1. A method of processing a first series of signals representing a stroke of a writing instrument on a writing surface to remove signals at the start of the stroke caused by retracing, said first series of signals corresponding to the X and Y coordinates of a series of points along the length of travel of the stroke, the method comprising: a. comparing said signals in said first series relative to each other to form a second series of signals, said second series of signals comprising only signals corresponding to local extrema points of said stroke, b. measuring the size of the angle defined by the first three local extrema points in said second series c. determining a reference angle which is related to the distances from the first local extrema point to the second local extrema point and from the second local extrema point to the third local extrema point, d. comparing the measured angle with the reference angle, and e. removing all signals in said first series corresponding to points before and second local extrema point if the measured angle is less than the reference angle.