About this site
This site is an interactive edition of Book I of Euclid's Elements, the book from which geometry was taught for more than two thousand years. It contains the 23 definitions, the 5 postulates and the 5 common notions, and all 48 propositions, from the construction of an equilateral triangle (I.1) to the Pythagorean theorem and its converse (I.47, I.48).
Every proposition has a figure you can drag. Move a point circled in red and the whole construction follows, so you can see that each step of the proof holds in every position, not only in the one drawn in the book. The proof plays step by step, and each step names the definition, postulate, common notion or earlier proposition it rests on. On the construction board you can try compass-and-straightedge constructions yourself.
Sources
The English text follows Sir Thomas L. Heath's translation (1908). The colours of the figures come from Oliver Byrne's 1847 edition, which printed Euclid's figures in red, yellow and blue. The other languages were translated for this site and have not yet been checked by native speakers, so corrections are very welcome.
Who makes it
This is an independent project. It is free to use and needs no account. Advertising helps pay for hosting. The source code is public on GitHub.
Questions and corrections: see the contact page.