A small molding or border, especially the semicircular molding that runs around the top of a column, or a bead in architecture.
From Greek 'astragalos' (ankle bone or knucklebone), because early astragals resembled the rounded shape of bones; later applied to architectural moldings with similar rounded profiles.
Ancient Greeks used knucklebones in games, and when architects created rounded moldings that looked similar, they literally called them 'bone shapes'—that's why architectural terms sometimes sound medical!
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