Gap fill

An alike pair with one cell between fixes the middle.

Technique 2 of 7 · Updated 12 October 2026

What is gap fill in Binairo?

Two of the same animal with a single gap between them: the gap must be the other animal. Gap fill is bookends' sibling: both come from the same three-cell window. Together they're the free moves you should exhaust before counting anything.

Pip: Owl, gap, owl. Whatever goes in the gap can't be an owl, or there'd be three in a row. Fill it, then count: three owls are placed, so the rest are foxes.

Interactive example · 1 of 2 · tap the empty cells

Pip: Same with foxes. Fox, gap, fox: the gap is an owl. Three foxes are placed now, so the last two cells are owls.

Interactive example · 2 of 2 · tap the empty cells

How to spot it fast

Look for the same animal two cells apart with a hole between them: A _ A. It's easy to miss on long lines, so trace each line once from end to end.

Common mistake

Treating a wider gap the same way. A _ _ A doesn't decide either cell on its own; you need the count or another clue.

Practise it

Unlimited puzzles let you drill any size. Other techniques: Pair → bookends · Count exhausted · Count pressure · Triple look-ahead · Links = and × · Chain reasoning · hard puzzles.

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