Geometry

Lecture notes from Texas Tech that build geometry from absolute axioms through Euclidean and non-Euclidean systems. Solid pick for math undergraduates moving past school geometry.

Razvan Gelca

Teacher edition of CK-12's open Basic Geometry text. Each chapter pairs with student materials and includes notes on common errors, pacing, and classroom activities.

Kate Dirga, Lori Jordan

College geometry course from the University of South Carolina that walks through Euclidean and non-Euclidean systems with Geometer's Sketchpad as a working tool. Strong on visual proofs and explorations.

University of South Carolina

Open textbook from City University of New York covering lines, angles, triangles, similarity, and trigonometry. The 2021 edition adds corrections and new diagrams, making it one of the most polished free college geometry books.

Henry Africk

Rigorous treatment of geometry built on Hilbert's axioms, starting from incidence and betweenness and reaching the parallel postulate in 256 pages. Best read after a first course in Euclidean geometry.

Oleg A. Belyaev