Locked candidates

Locked candidates, sometimes called pointing, are a classic sudoku move that killer cages set up constantly. If the digit 6 can only sit in one row inside a box, then 6 cannot sit in that row outside the box. The box has used up the row’s only chance to hold a 6.

Cages do the confining. A cage that must contain a 6, and that has only two cells inside a box, both on the same row, has pointed 6 along that row. You may not know which of the two cells it is. You do know it is one of them, so the other cells of the row lose 6. The same pattern runs down a column, and it runs in reverse: if 6 in a row is confined to one box, the rest of that box loses 6.

This is worth hunting whenever a unique cage sits across a box border. The two digits of a 16-cage (7 and 9) that both lie in the same box and the same row are pointing pairs. Erase 7 and 9 from the rest of the row and from the rest of the box. New singles fall out of that erasure more often than out of the cage itself.

Do not point a digit that still has a third home in the box, even if that home looks unlikely. Pointing is an all-or-nothing claim. One forgotten candidate makes the deletion illegal, and the puzzle will eventually contradict you. If you are using notes, the test is visual: in the box, the digit’s pencil marks occupy a single line.

Hard puzzles stack pointing on top of a 45-rule pair. The outie pair is 2 and 9. Both cells see the same row outside their box. Then 2 and 9 point. Players who stop when the outie sum is written down miss the second half of the step. The hint will highlight the line, not only the cage, when that is what matters.

Practice this on medium puzzles. They are rated to need pointing or a one-cell 45 step, and they still open with ordinary singles so you can see the locked digit arrive rather than having to invent it on an empty board.

Use this technique on a puzzle

Keep the combination table open while you read, and the rules if the cages themselves are still new.