Euclid is best known for his contributions to modern geometry, but he authored 13 books that covered not only geometry, but also mathematics, number theory and arithmetic. The math taught in most schools today is based on Euclid's findings, and he is widely known as the father of geometry.
Euclid's collection of books described all 465 propositions known to geometry as of 2015. All the propositions were derived from the 10 axioms that Euclid developed as truths. These axioms later became more commonly known as postulates. Half the postulates are directly related to geometry, while the other half are common, self-explanatory statements. It was theorized by Euclid that any person could understand his postulates and propositions because they are very simple. A common practice in Euclidian geometry is to understand standard definitions to avoid linguistic issues.
The reason Euclid has remained authoritative in the subject of geometry is because his explanations are very simple and easy to understand, while still being comprehensive. He is considered one of the greatest mathematicians of all time.
Euclid's work was so important that other mathematicians, such as Isaac Newton, used it in their findings.
In addition to exploring geometry and mathematics, Euclid wrote books about astronomy, optics and music theory.