Hard technique

Claiming Pairs: Box/Line Reduction Explained

Also called box/line reduction, or locked candidates (claiming).

Sometimes a row or column can fit a color only inside one block. The color must go in that block, so the rest of that block loses it.

Read the row

BEFORE

Five cells are open along the shaded row, and the block it passes through has three more. No cell shows a single color and there is no pair or triple to find, so read the row color by color and ask where each one can still go.

See where that color can go

Follow Light Purple along the row. Of the five open cells, only two can take it, and both sit inside the middle block. You do not need to know which one gets it. What matters is that this row’s Light Purple sits somewhere inside that block.

Rule Light Purple out of the block

Every row must place every color, so this row will put a Light Purple in one of those two cells. A block can hold only one Light Purple, so this block’s Light Purple is already on that row. The rest of the block loses the dot.

Read what that leaves

AFTER

One of those cells was showing Turquoise and Light Purple. With Light Purple gone, only Turquoise is left, so place it. Pointing starts in a block and clears cells along a line. Claiming starts in a line and clears cells inside a block.

Cells being readKey cellColor tints match the color being followed

Key Insight

Pointing and Claiming are the same move from opposite ends. With Pointing, the block can fit the color on only one line, so the rest of that line loses it. With Claiming, the line can fit the color in only one block, so the rest of that block loses it. Only one of the two can do real work at a time: if both are true of the same color, the cells that would lose it are the same cells that might take it, so there is nothing to clear. When a board stalls, try both directions on every color a row, column, or block is still missing.

If you’re stuck on a puzzle, see what to try next, in order.