Geometry which depends only on the first four of Euclid's Postulates and not on the Parallel Postulate.
Euclid himself used only the first four postulates for the first 28 propositions of the *Elements*, but
was forced to invoke the Parallel Postulate on the 29th.

**References**

Hofstadter, D. R. *Gödel, Escher, Bach: An Eternal Golden Braid.*
New York: Vintage Books, pp. 90-91, 1989.

© 1996-9

1999-05-25