Stationery items for sovereigns (6)

I believe the answer is:

rulers

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'sovereigns' is the definition.
(I've seen this in another clue)

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

Can you help me to learn more?

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

I've seen this clue in The Mirror.

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