Figures 2, 3, and 4 Figures 2, 3, and 4

Figure 2. Escher's "framework."

Figure 3. Triangle with angles
omega at
X1 and
X3, and
pi - omega at
X2.

Figure 4. The similar triangles
O1AC and
O2AB.
These figures are reprinted from "The Trigonometry of Escher's Woodcut "CircleLimit III" by H. S. M. Coxeter, which originally appeared in the
Mathematical Intelligencer, Vol. 18 No. 4 (1996), 42-46. ©Copyright 1996 Springer-Verlag New York. Reprinted by permission ofSpringer-Verlag New York, Inc. FURTHER REPRODUCTION IS STRICTLY PROHIBITED.