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tetraktus , uos, hê, ( [tetras] ) Pythagorean name for

A. the sum of the first four numbers, i.e. 10 (= 1+2+3+4), nai ma ton hameterai psuchai paradonta tetraktun, pagan aenaou phuseôs Carm.Aur.48 , cf. S.E.M. 7.94, TheoSm.p.94 H., Hierocl.in CA20p.465M.

II. the four terms (6:8:9:10) of the proportion corresponding to the chief musical intervals, Nicom.Exc.7, 10; also their sum +1 (= 36), ibid.; the sum of the first 8 numbers, Plu.2.381f.