Block Boundaries

Level: Intermediate

When a run of filled cells reaches a clue's length, block off both sides with X.

When the length of a run of filled cells matches the length of some clue block, that block is complete, so the cells immediately touching each end must be empty (X).

For example, if a line has only a single clue of 3 and three cells are already filled in a row, block off the cells to its left and right with an X to lock the block in place.

Marking these boundaries makes the separation from other blocks clear and tidies up the segments where the remaining clues can be placed, revealing the next move.

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