This is a page from the *Sharḥ ashkāl al-taʾsīs* (*Commentary on the Fundamental Theorems,* 1412-13) of Qāḍī Zāda Mūsā ibn Muḥammad, also known as al-Rūmī ("the Roman" or "the Byzantine") because he was born in Asia Minor in about 1364. He died in 1436. Qāḍī Zāda's book was a commentary on the *Ashkāl al-taʾsīs (Fundamental Theorems),* written by al-Samarḳandī (1250-1310) and containing a discussion of 35 of Euclid's propositions. The book shown in the image is a later copy of Qāḍī Zāda's work, probably written in the sixteenth century. At the top of the page is a discussion of Euclid's Proposition I-5, the "Bridge of Asses" proposition that the base angles of an isosceles triangle are equal. At the bottom, there is a discussion of Proposition I-6, the converse of I-5. Qāḍī Zāda was an astronomer and mathematician in the court of Ulugh Beg (1393-1449) in Samarkand. He and his colleagues compiled the first complete star catalogue since the time of Ptolemy.

*Posted 5/14/2008 (or earlier); updated 5/23/17*

See pages from two other copies of Qāḍī Zāda’s commentary on al-Samarḳandī’s *The Fundamental Theorems* (or *The Forms of Foundation,* based on Euclid’s *Elements*) in Mathematical Treasure: Commentary on *The Fundamental Theorems* and Mathematical Treasure: Qadi Zada’s Commentary on *The **Forms of Foundation.*