On the basis of their own research and that of other, Smyth and Silvers proposed that vision performs two distinct function in the control of handwriting. One function is to help the writer control the overall spatial arrangement of words on a horizontal line. We see an example of this function in figure 6.4, where handwriting samples taken from people writing without vision available show distinct deviations from a horizontal line. The second function for vision is to help the writer produce accurate handwriting patterns, such as the appropriate strokes and letters required for the written material. Again, evidence of this is seen in figure 6.4. People who wrote without vision available added or omitted strokes, added extra letters, deleted letters, and reversed letters.