Plural of astragal; small moldings in architecture, or knucklebones used in games and divination.
From Greek astragalos, which originally meant the bone of the ankle or a knucklebone. In architecture, the term transferred to a small convex molding because of its shape resembling these bones.
In classical architecture, you'll see tiny decorative ridges called 'astragals' that were named because they reminded builders of knucklebones—ancient architects were pretty poetic about geometry.
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