What was Euclid's contribution to mathematics that's still in use today

Euclid's most significant contribution to mathematics that is still in use today is his textbook, **The Elements**. This work systematically organized and presented the knowledge of geometry and number theory of his time, and it established a framework for mathematical proofs that is foundational in modern mathematics. The Elements has influenced the teaching and understanding of mathematics for centuries.

While his system of logic also had a profound impact, The Elements is particularly notable for its widespread influence on mathematical education and the development of geometry.