Proposition 29, Book XI, Euclid's Elements
This Demonstration is a dynamic illustration of Proposition 29, Book XI of Euclids's Elements.
Proposition 29, Book XI of Euclid's Elements states: Parallelepipedal solids which are on the same base and of the same height, and in which the ends of their edges which stand up are on the same straight lines, equal one another . ("Equal" means "have the same volume".)
 Euclid, Elements, T. L. Heath, trans., as The Thirteen Books of Euclid’s Elements, 2nd ed., Book 3, New York: Dover Publications, 1956 pp. 333–334.