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 08-20-2012, 04:05 PM #21 pvaudio Legend   Join Date: Jul 2009 Posts: 7,543 While your descriptions from a math standpoint are indeed correct, they only really apply to the classical definition of strokes. They don't apply to modern stroke mechanics. As a simple example, for both flat and "spin" shots, the racquet velocity should be perpendicular to the stringbed by your definition. If you hit a shot with underspin, as in a slice, this doesn't hold. Here, the racquet face is slightly open, so the strings are pointing slightly upward. The racquet velocity will be from high to low, so at impact, the racquet's velocity vector most certainly can be perpendicular to the string bed's normal vector. How? Easy, the ball is not going straight across the net, it needs elevation as well. I think what you meant to say is that for a flat ball, the racquet face must be normal to the direction of the racquet's travel through prior to and after impact. Because as I just pointed out, at impact, you can get any sort of spin you want and have the stringbed's normal vector be normal to the velocity vector.