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eighteenmo

noun
eigh·​teen·​mo
plural -s

Definition of eighteenmo 

: the size (approximately 4 by 6.5 inches) of a piece of paper cut 18 from a sheet also : paper or a page of this size —abbreviation 18mo —symbol 18°

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History and Etymology for eighteenmo

eighteen + -mo (as in duodecimo)

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