An instrument for drawing or testing right angles (1-6)

I believe the answer is:

t-square

(Other definitions for t-square that I've seen before include "Draughtsman's tool" , "Draftsman's instrument" , "Instrument used by draftsmen to draw parallel lines" , "Instrument for drawing right angles" , "Without denizens" .)

I've seen this clue in the Sydney Morning Herald.

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