Relating to or consisting of ten; a group, set, or division of ten items or units.
From Latin decenarius, from decem (ten). In English since the 16th century as both an adjective (relating to ten) and noun (a group of ten).
The Romans had decenary systems for everything—military organization, voting districts, even taxation—because ten was mathematically convenient and nobody had invented zero yet, so base-10 felt natural.
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