Abbess and abbot are both in this order (12)

I believe the answer is:

alphabetical

I'm afraid I can't explain this answer.

(Other definitions for alphabetical that I've seen before include "Ordered A to Z" , "One sort of order" , "One kind of order" , "Arranged in order from A to Z" , "Related to 26 characters" .)

I've seen this clue in The Independent.

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