It may also be interesting to notice, with regard to the movements by
which lines are produced, that they are divided into two systems; the
first is that of the wrist, which includes the horizontal, and lines in
nearly the same direction; the second is that of the fingers and thumb,
by which all other lines are formed. The following diagram exhibits the
two systems and their various lines grouped, and it will be observed
that there is a space marked (a) between the two sets, which may be
considered neutral ground. Lines in this direction may be produced by
either movement, as may be most convenient, but it will always be found
that these lines are the most trying to the hand.