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Hi everybody, After discussion a problem on mine on another thread,I think I better rephrase my problem. Given: A 3D model M and three point ids A,B,C of three points in the model M.The coordinates of all points in M are defined in global coordinate system. Problem: I want to define another coordinate system having its origin on point A.Then I need to express the coordinates of all points in M in terms of new coordinate system.Motivation is to find a representation of the geometry of points for a given model which is rotation invariant (doesnt change with rotation). My effort: I tried to define an orthogonal basis comprising vectors AB,X1 and X2 where X1 and X2 are orthogonal to each other and AB.Then I used standard change of basis technique by solving a linear system of equation as mentioned at http://www.gamedev.net/community/forums/topic.asp?topic_id=479364 The direction of axes of the new coordinate system seem to be ok but the origin of new coordinate system doesnt lie on A, rather it co-incides with the origin of global coordinate system,I guess. I need your thoughts on this issue please, This is driving me nuts now :( Best Regards, Justujo