Sudoku notes: how to make and use candidates correctly

A placed digit changes exactly three units: its row, column and box. Remove that digit from the notes in those units before you scan for a new pattern. This small habit prevents the most common notes error: a stale candidate that makes a cell look less constrained than it really is. If updating all notes at once feels slow, update the three affected units first, then return to the region where the move created the most pressure.

First, look for a cell with one candidate: a Naked Single. Next, look for a digit that appears in only one cell of a row, column or box: a Hidden Single. Then compare short note lists in a single unit. Two cells with the same two candidates form a Naked Pair; those digits can be removed from the other cells in that unit. The notes have done their job when they justify an elimination, not when they merely make the board look busy.

Use one stable order for candidates, such as ascending order from 1 to 9, and erase rather than overwrite. Avoid filling every empty cell with a dense wall of marks on an easy puzzle; start notes only when scanning no longer finds a move. On a screen, switch notes off temporarily when you need to inspect placed digits, then switch them back on for candidate work. Readability is not cosmetic: it is what lets you notice a pair or a single without missing it.

If several notes contradict the board or you cannot explain how they were made, stop and rebuild a small area instead of trying to repair every mark by memory. Check the last proven digit, recalculate one box or one nearly full row, and compare it with the current candidates. A solver can verify a copied puzzle, but it should not replace the reasoning step. The goal is a board whose notes you trust enough to act on.