Billiards

A somewhat related system is formed by a billiard ball bouncing around on a table. The issue of which sequence of horizontal and vertical sides the ball hits depends on the exact slope with which the ball is started (in the picture below it is 1/√2). In general, it is given by the successive terms in the continued fraction form (see page 914) of this slope, and is related to substitution systems (see page 903). (See also page 1022.)