Locked candidates and pointing pairs in sudoku
Locked candidates are the first technique that works on possibilities instead of placements, and the gateway to everything above easy. The idea fits in a sentence: when every home a digit has inside a box falls in one row or column, the digit is pinned to that line, and the line loses it everywhere else. Solvers call the pattern pointing pairs, pointing triples or claiming depending on the direction and the cell count. One habit covers all of it.
The pattern
Ask where a digit can still go inside one box. The top-left box below holds seven placed digits, leaving two open cells, and both sit in the middle row. The box’s 5 must land in one of them. Which one stays unknown, and the row does not care: either way, the 5 of that row lives inside the box.
The count takes seconds and the board backs it up: two open cells, one row, no third option. Nobody knows yet which cell takes the 5, and the elimination never needed to know.
The elimination
The row’s 5 sits inside the box, so every 5 penciled along the row outside the box points at a cell that can never take it:
Both struck cells collapse to a single candidate here, which shows why this technique carries the medium tier: a locked digit rarely solves its own box, and the cells it cleans elsewhere often fall on the spot.
Pointing and claiming
The move runs in both directions, and the names track the starting point. Pointing begins in the box: the box confines a digit to a line, and the line sheds it elsewhere, as above. Claiming begins in the line: when a row or column keeps all its homes for a digit inside one box, that box sheds the digit from its other cells. Read one direction and you have read both. The habit worth building is the question itself, asked box by box and digit by digit: where can this digit still go, and do the options share a line?
Near misses
The lock needs every home in one line. Same position, one placed digit removed:
One open cell in the wrong row is all it takes. The 5 might now land in the bottom row of the box, the middle row would go without, and the struck cells from before would need their 5s back. Before erasing anything, confirm the confinement: every home for the digit, inside the box, on one line. Auto notes makes the check honest, because a missing pencil mark can fake a confinement that does not exist.
Where it leads
Locked candidates carry our medium tier together with naked pairs, and every medium puzzle here needs one of the two at its hardest point. The box-and-line question also trains the exact eye that X-wings and swordfish demand later, where the confinement spans rows and columns instead of a single box. Learn to ask where a digit is trapped, and the upper tiers open from there.
Frequently asked questions
What are locked candidates in sudoku?
When every cell that can hold a digit inside one box falls in a single row or column, the digit is locked to that line. It must land there somewhere, so you can erase it from the rest of the line outside the box. The reverse works too: a digit confined to one box within a line disappears from the rest of that box.
What is the difference between pointing and claiming?
Direction. Pointing starts in the box: the box confines the digit to a line, so the line loses the digit elsewhere. Claiming starts in the line: the line confines the digit to a box, so the box loses it elsewhere. Both are the same idea read from opposite ends, and most solvers stop distinguishing them after a while.
Are pointing pairs the same as locked candidates?
Yes. Pointing pairs and pointing triples name the box-to-line case by how many cells hold the digit, two or three. Locked candidates is the umbrella term for the whole family, claiming included. Different sources pick different names for the same move.
When do I need locked candidates?
Our medium tier is where they first appear: every medium puzzle needs locked candidates or a naked pair at its hardest point. They are the first technique that works on possibilities instead of placements, which makes them the natural next step once singles feel automatic.