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.