Euclid is a WHO. Euclid of Alexandria was a Greek mathematician, often referred to as the "Father of Geometry". He was active in Alexandria during the reign of Ptolemy I - approximately 330 B.C. to 260 B.C.
Many scholars worked and taught in Alexandria, and that's where Euclid wrote The Elements. The Elements is divided into thirteen books which cover plane geometry, arithmetic and number theory, irrational numbers, and solid geometry.
Euclid is a short geometry primer that includes a few of the most basic and well known principles of plane geometry with emphasis on practical use, enjoyment, and beauty.
The topics are presented in simple, clear, visual, informal terms; no attempt is made to be "mathematically rigorous." Each topic includes a self quiz and a final exam is offered at the end of "the course".