To adjoin something is to be next to it or share a border with it. It is often used for rooms, lands, or buildings that touch each other.
From Old French 'ajoindre' and Latin 'adjungere', meaning 'to join to', from 'ad-' (to) + 'iungere' (to join). It has always carried the sense of physical joining.
Adjoin is like the verb form of 'neighbors'—two spaces that literally touch. When hotel rooms adjoin, it means the wall between them is all that keeps two lives separate.
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