Completed blocks
A run you've finished is worth more than the squares it fills — it also tells you which neighbouring squares must be empty. "Capping" a completed block, and recognising when two filled squares belong to the same run, are two of the most reliable ways to keep a solve moving. This guide covers both, and the one mistake to avoid. It assumes you're comfortable with runs, clues, and marking empties.
Cap a completed run
When a run reaches its full clue length, it can't grow any further — so the squares immediately touching each end must be empty. Marking those squares (capping the run) is what separates it from its neighbours and stops any other run from accidentally merging into it.
In the example below, the line's clue is 3 2 and the run of 3 has been placed. Because that run is already at its full length, the squares on either side of it are capped with an ✕ — and those caps are exactly what the run of 2 and the crossing lines need next.
Capping a completed run of 3
The run of 3 is at full length, so the squares on both sides are capped empty (✕) — separating it from the run of 2 still to come.
Join two cells of the same run
When a line has a single run and you've found two filled squares some distance apart, they must belong to that one run — so everything between them is filled too, and both ends can be capped. This "joining" move often reveals several squares at once from just two known cells.
Here the clue is a single 4, and two squares four apart are already filled. A run of 4 covering both can only sit in one place, so the two squares between them fill in and both ends are capped empty.
Joining two cells of a single run of 4
Two filled squares four apart must be the same run of 4: fill the two between them, then cap both ends (✕).
Why capping unlocks the crossing lines
The empty caps you place aren't just tidiness. Each ✕ at the end of a completed block is a fresh fact for the perpendicular line running through it — it tells that row or column that no run can occupy the square there. A block you complete in a row very often makes the next move possible in a column, and vice versa. Capping is how progress spreads across the grid.
Don't cap a run before it's complete
The one trap: only cap a block once you've proven it's at full length. A filled square isn't necessarily the end of its run — the run might still extend further, and capping it prematurely plants a wrong empty mark that quietly corrupts the puzzle. Cap when a run has reached its clue's number exactly, or when a boundary forces it, never on a hunch.