Jacobian Invariant I'm trying to find an on-line source for the "Jacobian Invariant" of four points on a line. The cross-ratio depends on the order of the points, while this one doesn't. I've done several Google searches and can't seem to find anything. Does anyone know of a good source off-hand?

The commonly used name is $j$-invariant. See here or here (you will have to read a little to realize that they are talking about the same thing!)