Hopefully this should be a simple problem to solve! I have two verticies v1 & v2 which define a 3D line (a line-of-sight). I want the length of the line be constant; so when (for example) the vertex v1 changes, I want to adjust v2 so that the lines length is back to its default value. I can think of a couple of ways of doing this (using the delta value of the line-slope to move the a vertex along it`s length, etc), but this seems very in-elegant, and I`m sure that there is a much better (& simpler!) way of achieving this. I`m sure that I`ll be kicking myself when someone points out the solution, (and folly!), but hey, what are forums for, eh?! Thanks in advance, BenE.