sesquialteral, a. [L. sesquialter once and a half; sesqui- + alter other: cf. F. sesquialtére.].
Once and a half times as great as another; having the ratio of one and a half to one. [1913 Webster]
Sesquialteral ratio (Math.),
the ratio of one and a half to one; thus, 9 and 6 are in a sesquialteral ratio.