Book excerpt: The study of the arithmetical properties of triangles dates back to ancient Greece, and possibly beyond. This classic text, written by a distinguished mathematician and teacher, focuses on a fundamental cornerstone of elementary geometry, the theorem of Pythagoras, and its applications. Unabridged republication of the edition published by the Graduate School of Science, Yeshiva University, New York, 1962. Translated by Dr. Ambikeshwar Sharma.