Monarchs’ measuring devices (6)

I believe the answer is:

rulers

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'monarchs'' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for rulers that I've seen before include "Drawing sticks or governors" , "Drawing sticks or monarchs" , "Strips with straight edge to measure" , "Kings, queens, presidents etc" , "they must not be crooked" .)

I've seen this clue in The Hindu.

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