Hi Charlie, To set a model's transformation to the identity: identity = chimera.Xform() model.openState.xform = identity This may throw the model out of view. To apply a rotation about the z-axis in the model's coordinate system: zrot = chimera.Xform.zRotation(30) # rotation by 30 degrees model.openState.localXform(zrot) To apply a rotation about the screen z-axis: zrot = chimera.Xform.zRotation(30) # rotation by 30 degrees model.openState.globalXform(zrot) The localXform() and globalXform() methods just multiply the current transformation by the given one (on the right or left) and set the model's transformation to the result. The model transformation maps a point in the model coordinate system to the screen coordinate system (x horizontal, y vertical, z out of screen) controlling where the model appears on the screen. When you use localXform(xf) the xf gets applied first, and then the current transformation. When you use globalXform(xf) the current transform is applied first and then xf. Some more examples are given in the Programmer's Guide FAQ: http://www.cgl.ucsf.edu/chimera/docs/ProgrammersGuide/faq.html#q4 Getting the transformation math right can be tricky. Tom