*sesquialterate*

{ } 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. [ 1913 Webster ]