Use this guide as a working method, not a list of tricks. Read one section, apply it to a real activity, and keep the evidence that shows why each move works.
The short answer
The practical answer to locked candidates sudoku is to use one observable method, keep a note of what has already been reviewed, and make the next selection from support on the page. For beginner-to-intermediate Sudoku solvers who already understand candidate notes and want a reliable bridge from singles into harder elimination techniques, the main objective is to recognize both pointing and claiming locked-candidate patterns, prove which candidate eliminations are valid and use the resulting reductions to reveal singles, pairs or other easier deductions without guessing. That is more reliable than moving faster, copying someone else's decision or opening the answer material as soon as the exercise becomes uncertain.
A good practice period starts with a partly solved Sudoku with current candidate notes, one digit selected for scanning, observable three-by-three box boundaries and an erasable or reversible way to remove candidates as each box-line relationship is verified. Those details sound ordinary, but they decide whether the participant can see the information, reverse a mistake and understand the next step. The task should feel demanding because of its intended thinking skill, not because the page is cramped, the instructions are hidden or the controls are difficult to operate.
Start with Medium Sudoku when you want to test the setup on a complete activity rather than an isolated example.
Use the method in this walkthrough on one complete sudoku exercise before collecting extra tips. Consistently applying a small set of habits produces a clearer improvement than applying a long list at random. When the task is complete, explain which clue or observation justified the final actions; that explanation is the best check that the method can be used again.
Understand what the activity is asking
Before starting, describe the task in one sentence without relying on its theme or decoration. For locked candidates in sudoku: pointing and claiming explained, the action is to recognize both pointing and claiming locked-candidate patterns, prove which candidate eliminations are valid and use the resulting reductions to reveal singles, pairs or other easier deductions without guessing. Naming that action keeps attention on the skill the page is meant to practise instead of on whichever detail happens to look most noticeable.
Next, find what counts as support in this format: the digits already present in the same row, column and box, together with the candidates those digits eliminate. A choice that merely looks promising is not equivalent to one supported by the exercise. Teaching that distinction helps a beginner pause without feeling stuck and helps an experienced participant catch a careless assumption before it spreads.
Define completion before the first mark. Here, completion means that all 81 cells are filled and every row, column and 3×3 box contains the digits 1 through 9 exactly once. A observable endpoint makes progress easier to judge and stops the exercise from ending only because time or attention has run out.
Set up before you begin
Prepare a partly solved Sudoku with current candidate notes, one digit selected for scanning, observable three-by-three box boundaries and an erasable or reversible way to remove candidates as each box-line relationship is verified. Put the task on a stable surface and remove unrelated sheets from the immediate workspace. On screen, use a comfortable zoom and make sure the main controls remain observable. On paper, keep an eraser or a second marking color available so tentative thinking does not become indistinguishable from a confirmed answer.
Read the title, instructions, legend and any word bank or clue list before making the first mark. This preview is not wasted time. It reveals the size of the task, the notation used by the author and any special condition that changes the obvious plan. It also gives the participant a chance to ask about unfamiliar vocabulary before that vocabulary is confused with task difficulty.
Pick a simple recording convention. Confirmed work should look different from a possibility, a tested branch or a discarded answer. Consistent marks reduce working-memory load because the page remembers earlier judgements. For group play, agree on the same convention aloud so one person's mark is meaningful to everyone else.
- Keep the full task and its instructions observable.
- Use reversible marks for uncertain work.
- Separate the participant sheet from the answer material.
- Agree what a finished and reviewed result looks like.
A step-by-step method
Step 1: scan one three-by-three box at a time for a candidate digit that remains possible only in cells lying on the same row or the same column; when every possible location for that digit in the box shares one line, the digit is locked to that line inside the box. Do this deliberately on the first attempt and say what you are verifying. The aim is not to make every decision slow; it is to make the reasoning repeatable. If this pass produces no answer, note which area was reviewed and continue. A complete negative scan still reduces uncertainty.
Step 2: apply pointing logic by keeping the candidate in the aligned cells inside the box and removing that candidate from other unsolved cells farther along the same row or column outside the box, because the box must place the digit in one of its aligned positions. On the first practice page, pause after this step and describe the support in one sentence. That small explanation separates a justified selection from a lucky one. If the support is incomplete, leave the mark reversible and decision to a part of the exercise that can be confirmed.
Apply these first two decisions in Hard Sudoku; leave the page open beside this section and return only when you can explain the evidence for the next step.
Step 3: apply claiming logic in the reverse direction by scanning a row or column where all remaining positions for one candidate fall inside the same box; the digit must be placed somewhere in that box-line intersection, so remove the candidate from the other cells of that box outside the line. On the first practice page, pause after this step and describe the support in one sentence. That small explanation separates a justified selection from a lucky one. If the support is incomplete, leave the mark reversible and decision to a part of the exercise that can be confirmed.
Step 4: verify the scope before every elimination: pointing starts from a box and eliminates along the line outside the box, while claiming starts from a row or column and eliminates elsewhere inside the box; never delete the candidate from the cells that prove the lock. Keep the scope of this step narrow enough that it can be finished and reviewed. Work from one edge, unit, clue group or caller note to the other, then mark the pass as complete. Order matters because it prevents attractive details from pulling attention back to places already reviewed.
Step 5: after each valid elimination immediately rescan the affected row, column and box for naked singles, hidden singles or newly exposed pairs, because the practical value of locked candidates is usually the easier deduction they create next. Keep the scope of this step narrow enough that it can be finished and reviewed. Work from one edge, unit, clue group or caller note to the other, then mark the pass as complete. Order matters because it prevents attractive details from pulling attention back to places already reviewed.
A worked practice example
Consider this example: if candidate 4 appears only in two cells of the top-left box and both cells are on row three, 4 is locked to row three inside that box; remove candidate 4 from other unsolved cells on row three outside the box, then check whether any affected cell or unit has been reduced to a single legal option. Restate the exact target and find the information already supplied by the page before making a permanent mark. Point to the first practical constraint and name the alternatives it removes. That creates a short chain of reasoning another person can follow and challenge.
Use the first two steps in order. First, scan one three-by-three box at a time for a candidate digit that remains possible only in cells lying on the same row or the same column; when every possible location for that digit in the box shares one line, the digit is locked to that line inside the box. Next, apply pointing logic by keeping the candidate in the aligned cells inside the box and removing that candidate from other unsolved cells farther along the same row or column outside the box, because the box must place the digit in one of its aligned positions. If the support conflicts with an early hunch, keep the support and discard the hunch. The aim is a result that survives every relevant check, not a defence of the first idea that came to mind.
For the third step, apply claiming logic in the reverse direction by scanning a row or column where all remaining positions for one candidate fall inside the same box; the digit must be placed somewhere in that box-line intersection, so remove the candidate from the other cells of that box outside the line. Pause and inspect the surrounding information instead of staring only at the current detail. Many errors begin when a locally attractive choice hides a condition elsewhere on the page. One wider check is faster than repairing several dependent choices later.
Finish with the verification that belongs to this format: scan every row, column and box for duplicates, then assess the finished grid with the recorded solution. This final pass turns a plausible result into a completed exercise and gives the participant a precise place to look if something does not agree.
How to adjust the difficulty
Change one variable at a time. For this format, the variables that genuinely affect difficulty are clue count, candidate density, the solving techniques required and how many logical steps must be chained together. If several change together, it becomes difficult to know which one made the exercise more accessible or more demanding.
For a gentler version, keep the same theme but narrow the number of competing possibilities. Concentrate on one action: scan one three-by-three box at a time for a candidate digit that remains possible only in cells lying on the same row or the same column; when every possible location for that digit in the box shares one line, the digit is locked to that line inside the box. Use generous spacing, a shorter set or a partly completed example. Support should expose the reasoning while leaving the next selection to the participant.
For a stronger challenge, preserve the same basic conditions and ask for this action without prompting: verify the scope before every elimination: pointing starts from a box and eliminates along the line outside the box, while claiming starts from a row or column and eliminates elsewhere inside the box; never delete the candidate from the cells that prove the lock. Add complexity only after the participant can explain the easier method. Productive difficulty creates more meaningful judgements; poor difficulty merely adds tiny print, ambiguous wording or hidden instructions.
To compare the change at a nearby level, open Easy Sudoku and change one difficulty variable instead of changing the whole routine at once.
Watch behavior instead of relying only on the final score. Repeated random marks, long periods without a plan or refusal to check work signal that the level may be too high or the method unclear. Fast completion with no explanation may signal that the level is too low. Adjust the next task, not the participant's confidence.
Common mistakes and what to do instead
Avoid calling a candidate locked when the digit still has a possible position elsewhere in the same box off the shared row or column. A good recovery is smaller than a restart. Assess the questionable choice with the instructions and shared note, explain exactly why it fails, then correct that one branch. Keeping the rest of the valid work protects both confidence and practical information.
Avoid confusing pointing with claiming and eliminating from the wrong region of the grid. When this happens in a group, describe the mismatch without blaming the person who made the decision. Reconstruct the support together, agree on the earliest reliable state, and resume with one participant acting while another verifies the governing condition.
Avoid removing the candidate from the aligned cells that establish the lock instead of preserving those candidate positions. This often appears when speed replaces verification. Do not erase the whole page. Isolate the row, route, clue, card or turn where support disappeared, remove the unsupported mark, and recheck the neighboring information that depended on it.
Avoid using stale or incomplete pencil marks, which can make a false box-line restriction appear valid. This often appears when speed replaces verification. Do not erase the whole page. Isolate the row, route, clue, card or turn where support disappeared, remove the unsupported mark, and recheck the neighboring information that depended on it.
Avoid finding a legitimate locked-candidate elimination and then continuing to hunt for advanced patterns without rescanning for the new single or pair it may have exposed. Pause at the first observable contradiction and return to the last confirmed point. Name the condition that should control the next selection, restore only the information affected by the error, and continue from support rather than from the same hunch.
Build a checklist for Locked Candidates in Sudoku: Pointing and Claiming Explained
Turn the method into a short checklist that can sit beside the exercise. Write the target first: recognize both pointing and claiming locked-candidate patterns, prove which candidate eliminations are valid and use the resulting reductions to reveal singles, pairs or other easier deductions without guessing. Under that line, note the support that belongs to this format—the digits already present in the same row, column and box, together with the candidates those digits eliminate. This prevents a checklist from becoming a generic reminder to 'be careful'; every item should point to something the participant can actually see, say or verify.
Make the first checkpoint specific: scan one three-by-three box at a time for a candidate digit that remains possible only in cells lying on the same row or the same column; when every possible location for that digit in the box shares one line, the digit is locked to that line inside the box. The second checkpoint is different: apply pointing logic by keeping the candidate in the aligned cells inside the box and removing that candidate from other unsolved cells farther along the same row or column outside the box, because the box must place the digit in one of its aligned positions. Keep both as complete actions rather than reducing them to one-word labels. A participant returning after a pause should be able to read either line and know exactly what to inspect next without reconstructing the whole article.
Use the middle of the checklist for the judgements most likely to be skipped: apply claiming logic in the reverse direction by scanning a row or column where all remaining positions for one candidate fall inside the same box; the digit must be placed somewhere in that box-line intersection, so remove the candidate from the other cells of that box outside the line. Follow that with verify the scope before every elimination: pointing starts from a box and eliminates along the line outside the box, while claiming starts from a row or column and eliminates elsewhere inside the box; never delete the candidate from the cells that prove the lock. Put a small box beside each action, but tick it only after the action is complete. The boxes are a note of work already performed, not a score and not a substitute for explaining why the result is valid.
Add one recovery note based on the most relevant errors: avoid calling a candidate locked when the digit still has a possible position elsewhere in the same box off the shared row or column, and watch for confusing pointing with claiming and eliminating from the wrong region of the grid. If either appears, return to the most recent confirmed support instead of restarting the complete exercise. Finish the checklist with the final action—after each valid elimination immediately rescan the affected row, column and box for naked singles, hidden singles or newly exposed pairs, because the practical value of locked candidates is usually the easier deduction they create next—and the format check: scan every row, column and box for duplicates, then assess the finished grid with the recorded solution.
Test the checklist against the worked situation rather than reading it in the abstract: if candidate 4 appears only in two cells of the top-left box and both cells are on row three, 4 is locked to row three inside that box; remove candidate 4 from other unsolved cells on row three outside the box, then check whether any affected cell or unit has been reduced to a single legal option. Cover the article, follow the written checkpoints and notice where the checklist leaves you unsure. Rewrite that line with the missing object, location or stopping condition. A practical checklist becomes shorter as its actions become more precise; it does not become a second copy of the tutorial.
Store the checklist with a partly solved Sudoku with current candidate notes, one digit selected for scanning, observable three-by-three box boundaries and an erasable or reversible way to remove candidates as each box-line relationship is verified. On the next attempt, use it only at the beginning, after a contradiction and during final verification. If the participant can carry out after each valid elimination immediately rescan the affected row, column and box for naked singles, hidden singles or newly exposed pairs, because the practical value of locked candidates is usually the easier deduction they create next without looking back at the article, the checklist is supporting independent work. If it repeatedly leads to removing the candidate from the aligned cells that establish the lock instead of preserving those candidate positions, revise the wording before increasing the difficulty.
Before sharing the checklist, run one final failure test. Ask whether it prevents using stale or incomplete pencil marks, which can make a false box-line restriction appear valid and whether a new participant could still fall into finding a legitimate locked-candidate elimination and then continuing to hunt for advanced patterns without rescanning for the new single or pair it may have exposed. Add one concrete warning only when it changes an action; vague cautions create visual noise. The finished checklist should explain what to do, what support confirms it and when to stop, correct or continue.
- Target: recognize both pointing and claiming locked-candidate patterns, prove which candidate eliminations are valid and use the resulting reductions to reveal singles, pairs or other easier deductions without guessing.
- First action: scan one three-by-three box at a time for a candidate digit that remains possible only in cells lying on the same row or the same column; when every possible location for that digit in the box shares one line, the digit is locked to that line inside the box.
- Independent check: verify the scope before every elimination: pointing starts from a box and eliminates along the line outside the box, while claiming starts from a row or column and eliminates elsewhere inside the box; never delete the candidate from the cells that prove the lock.
- Final verification: scan every row, column and box for duplicates, then assess the finished grid with the recorded solution.
Teaching, family and group use
When using this walkthrough with beginner-to-intermediate Sudoku solvers who already understand candidate notes and want a reliable bridge from singles into harder elimination techniques, model one complete selection rather than completing the whole exercise. Say what you notice, what you can eliminate and why the remaining step is justified. Then hand control back. The learner needs to practise choosing the support, not merely watch a polished solution.
Use questions that reveal process: What have you reviewed? Which condition applies here? What would make this answer impossible? Where will you look next? These prompts are more practical than asking whether the participant is sure. They allow help without converting the exercise into a sequence of hints supplied by the adult or host.
If the learner needs another explanation of the same format, continue with How to Play Sudoku: A Beginner’s Guide before supplying an answer.
In pairs, assign roles that rotate. One person can point or mark while the other checks the condition, clue list or caller note. Switch after a small milestone. This prevents one confident participant from taking every decision and gives both people a concrete responsibility.
End with a one-minute reflection. Ask which plan found the most information, where the first error appeared and what the participant will do first on the next exercise. Keep the reflection specific to observable actions. General praise feels pleasant; specific feedback makes the skill portable.
A focused 15-minute practice routine
Minute 0–2: preview the page, read the conditions and pick the recording convention. Name the method you will practise. This short setup keeps the practice period from becoming an unstructured race and makes later reflection possible.
Minute 2–7: carry out two actions in order. First, scan one three-by-three box at a time for a candidate digit that remains possible only in cells lying on the same row or the same column; when every possible location for that digit in the box shares one line, the digit is locked to that line inside the box. Next, apply pointing logic by keeping the candidate in the aligned cells inside the box and removing that candidate from other unsolved cells farther along the same row or column outside the box, because the box must place the digit in one of its aligned positions. Do not measure speed yet. Put a small neutral mark beside any selection that remains uncertain, then continue with information that can be confirmed.
Minute 7–12: decision to the harder area. Use this check: apply claiming logic in the reverse direction by scanning a row or column where all remaining positions for one candidate fall inside the same box; the digit must be placed somewhere in that box-line intersection, so remove the candidate from the other cells of that box outside the line. Then apply this follow-up: verify the scope before every elimination: pointing starts from a box and eliminates along the line outside the box, while claiming starts from a row or column and eliminates elsewhere inside the box; never delete the candidate from the cells that prove the lock. If the exercise is complete, verify it instead of opening another page immediately. If it is unfinished, stop at a clear point and summarize what remains so the practice period can resume without repeating work.
Minute 12–15: check the answer material only after recording a final result or a specific unresolved question. Assess methods, not just answers. Pick the next task at the same level if the reasoning was inconsistent, or one neighboring level harder if the method felt controlled.
Print, screen and accessibility choices
Use the original PDF for printing because it preserves the intended grid proportions and line weight. Print one proof at actual size before a batch. On a narrow A4 printer, inspect the preview carefully rather than accepting automatic shrinkage. Keep answer pages out of participant sets unless self-checking is part of the exercise.
On screen, increase browser zoom until text is comfortable, then validate that controls remain reachable and the task itself still fits the available space. Use keyboard, touch or mouse according to the participant's motor comfort. Reduced motion should not remove status messages or progress information.
For visual tracking, use strong light without glare and introduce a ruler edge, reading strip or focus mode when the page becomes difficult to follow. Preserve the support that matters for this format—the digits already present in the same row, column and box, together with the candidates those digits eliminate—at a comfortable size and contrast. Accessibility changes how information is presented; it does not need to remove the reasoning that makes the exercise satisfying.
How to know the method is working
The clearest sign of improvement is not a single fast time. Look for fewer unsupported actions, cleaner corrections, more complete scans and explanations that refer to the actual conditions. A participant who can recover from an error has learned more than a participant who happened to guess the right branch.
Repeat a similar sudoku exercise within a day or two and begin without rereading the whole walkthrough. If the participant independently uses at least two of the method steps, the routine is becoming available from memory. If not, shorten the next practice period and practise one step in isolation.
Keep a tiny practice note with the task title, level, time spent, plan used and one sentence about the hardest selection. Do not turn the note into a public ranking. Its purpose is to pick the next practical exercise and to make gradual progress observable.
Test the skill on a fresh puzzle
Use one of the fresh sudoku practice pages linked in this walkthrough rather than repeating the worked example. Before starting, write down two actions: scan one three-by-three box at a time for a candidate digit that remains possible only in cells lying on the same row or the same column; when every possible location for that digit in the box shares one line, the digit is locked to that line inside the box. Then, apply pointing logic by keeping the candidate in the aligned cells inside the box and removing that candidate from other unsolved cells farther along the same row or column outside the box, because the box must place the digit in one of its aligned positions. Put the article aside and try to recognise the right moment for each action without copying the demonstration wording.
After the second page, assess one successful selection with one moment of uncertainty. Ask whether this step supplied enough support—apply claiming logic in the reverse direction by scanning a row or column where all remaining positions for one candidate fall inside the same box; the digit must be placed somewhere in that box-line intersection, so remove the candidate from the other cells of that box outside the line—and whether the participant avoided this common error: calling a candidate locked when the digit still has a possible position elsewhere in the same box off the shared row or column. Note one specific adjustment for the next attempt. That comparison turns locked candidates sudoku into a transferable skill instead of a one-page performance.
Frequently asked questions
What is the best first step for locked candidates sudoku?
Preview the complete exercise, then make the first action explicit: scan one three-by-three box at a time for a candidate digit that remains possible only in cells lying on the same row or the same column; when every possible location for that digit in the box shares one line, the digit is locked to that line inside the box. This gives you an ordered starting point and reduces the temptation to make a random mark before the conditions and available information are clear.
Should I use a timer?
Use a timer only after the method is accurate. A quiet elapsed-time display can help assess your own sessions, but a countdown often encourages guesses, incomplete verifying and avoidable motor errors. For children and new players, finish one logical milestone instead of racing a clock.
When should I look at the answer?
Look after you have a complete result or a precisely stated point of uncertainty. First note what you tried and which condition stopped progress. Then assess the relevant part of the answer material. This makes verifying a learning step rather than a substitute for solving.
How do I make a sudoku exercise easier without giving away the answer?
Reduce one source of complexity and keep the core condition. You might provide larger print, fewer choices or a shorter set, then demonstrate one action: apply pointing logic by keeping the candidate in the aligned cells inside the box and removing that candidate from other unsolved cells farther along the same row or column outside the box, because the box must place the digit in one of its aligned positions. Ask the participant to make the next selection independently.
How often should I practise?
Two or three focused sessions each week are enough for most recreational solvers. Stop while the participant can still explain the method. Short, repeated practice with a nearby difficulty is more practical than one long practice period that ends in fatigue and random guessing.
Can this method be used on paper and online?
Yes. The reasoning is the same. Online controls can save progress and provide reversible input, while paper makes annotations and group discussion easy. Pick the format that keeps the task readable and lets the participant distinguish tentative work from confirmed answers.
