The BSHM logo derives from the proof of the Pythagorean theorem given by Oliver Byrne in his 1847 edition of Euclid: The First Six Books of The Elements of Euclid in which Coloured Diagrams and Symbols are Used instead of Letters for the Greater Ease of Learning. An original copy of Byrne’s Euclid is in the BSHM Fauvel Collection held in the Open University Library, Milton Keynes, UK.  A digital edition of Byrne’s Euclid can be found here. The Pythagorean theorem is on pages 48–49.