τετρακτύς

A. the sum of the first four numbers, i.e. 10 (= 1+2+3+4), ναὶ μὰ τὸν ἁμετέρᾳ ψυχᾷ παραδόντα τετρακτύν, παγὰν ἀενάου φύσεως Carm.Aur. 48, cf. S.E. M. 7.94, TheoSm.p.94 H., Hierocl. in CA 20p.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.