How lines come together in square (2,5,6)

I believe the answer is:

at right angles

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'square' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for at right angles that I've seen before include "Perpendicularly" , "Vertically" , "Perpendicular" .)

I've seen this clue in The Irish Times.

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