Hard technique

Claiming Pairs: Box/Line Reduction Explained

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

When a color can only go inside one block along a row, the rest of that block cannot have that color. The same works for columns.

Read the row

BEFORE

The shaded row has five open cells. The middle block has three more. No cell shows a single color, so nothing can be placed yet. Follow one color along the row.

See where Light Purple can go

Light Purple can only go in two cells of the row, and both are in the middle block. One of them will be Light Purple. Either way, the row’s Light Purple is in that block.

Remove Light Purple from the rest of the block

A block can only have one Light Purple. It will be in the shaded row, so no other cell in the block can be Light Purple. Two cells lose their Light Purple dot.

Place what is left

AFTER

The bottom right cell of the block had Turquoise and Light Purple. With Light Purple gone, only Turquoise is left. Place it.

Cells being readKey cellColor tints match the color being followed

Key Insight

Pick a color a row still needs. Find the cells in the row where it can go. If they all sit in one block, remove that color from the rest of the block. It works the same for columns.

Related Techniques

Pointing PairsThe same move in reverse. When a block can fit a color only along one row or column, the rest of that line loses it.

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