Number Loom: trace a conflict back to an editable decision
Compare an obvious duplicate with a locally legal mistake on one four-by-four board, then use Clear and Undo without blaming fixed clues for the conflict.
Worked examples use the Standard board unless a different size is stated. Hard and Expert may change the board dimensions or amount of information to track.
A highlighted square is not automatically the mistake
Number Loom marks duplicates in each row, column and outlined box. When an editable number conflicts with a fixed clue, both participating squares may be highlighted. That does not mean both values are wrong. The fixed clue is part of the puzzle’s starting constraints and cannot be edited; the useful question is which later decision introduced the disagreement.
Begin with a separate Standard-size teaching board: 0 2 0 4 / 3 0 1 0 / 2 0 0 1 / 4 1 0 0. Zero denotes an empty square, and the boxes are two by two. The examples below examine this same position independently, rather than suggesting that every newly dealt board will look identical.
Follow a duplicate to its cause
Enter 2 at row 1 column 1. It now duplicates the fixed 2 in row 1 column 2, and also the fixed 2 in row 3 column 1. Several squares participate in the conflict, but the newly entered value caused it. Removing that editable 2 resolves both disagreements without changing either clue.
Clear removes the selected editable number. Undo instead reverses the most recent accepted entry, which is convenient if the mistake was your last action. These are different operations when you have edited other squares in between. Inspect the selected row and column before clearing anything; correcting an old value should not require dismantling unrelated, well-supported work.
A wrong entry can initially have no duplicate
Return to the original teaching position and consider putting 3 at row 4 column 3. That row currently contains 4 and 1, its column contains a fixed 1, and its box also contains a fixed 1. None already contains 3, so this entry creates no immediate duplicate. It is nevertheless inconsistent with a completed solution.
To see the problem without an answer reveal, inspect row 1 column 1. Its column contains 3, 2 and 4, so it must be 1. Row 1 then needs 3 at column 3, but your tentative 3 at row 4 column 3 prevents it. The contradiction emerges from a chain of constraints rather than an immediate repeated value.
Return to a justified decision point
The conflict display is therefore a check on duplicates, not a certificate that every unmarked entry is correct. When a chain becomes impossible, identify the unsupported decision behind it. If that decision was recent, Undo can return through its consequences. If several independent edits followed, Clear the relevant editable square and recheck the affected row, column and box.
Avoid treating all filled squares as equally certain. A number forced by existing constraints has a different justification from one entered to explore a possibility. There is no built-in annotation layer or automatic explanation of that distinction. Keep a brief external record if needed, and use Restart when you want the same original clues with all your entries removed. The goal is to recover sound reasoning, rather than simply making the highlighting disappear.
An original guide for this site's implementation. Found an unclear step? Let us know.