sesquĭ-tertĭus, a, um, num. adj., corresp. to the Gr. ἐπίτριτος,
containing one and a third , or four thirds; bearing the ratio of four to three : intervallum, Cic. Univ. 7, 21; cf. sesquioctavus, and v. sesqui.
sēsqui-tertius num. adj., containing one and a third, of four thirds: intervallum. ...
An Elementary Latin Dictionary