Second harmonic distortion  --  A straight arm forms a tangent at one point in the groove, but if an offset is formed by bending it near the cartridge, two tangential points can be obtained at which tracking error is zero. Some designs equally space the zero points from the start and finish of the groove to equalize maximum tracking error either side of and in between the two points. However, distortion for a given tracking error increases as the groove velocity decreases, and so is at a maximum near the centre of the disc. It is better, therefore, to arrange the second point to be nearer the disc centre so that the distortion is equalized rather than the tracking error. For a 12 inch disc which starts at 145 mm and finishes at 60 mm the optimum distances are 120 mm and 65.6 mm.