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

Euclid, often referred to as the "Father of Geometry," made significant contributions to mathematics, most notably through his work "Elements." Written around 300 BCE, "Elements" is a comprehensive compilation of the knowledge of geometry and mathematical principles of his time. Its main contributions include:

1. **Axiomatic Method**: Euclid's approach involved starting from a small set of axioms or postulates and deriving a wide range of propositions and theorems. This method laid the groundwork for the rigorous proof system used in mathematics today.

2. **Geometry**: "Elements" covers plane geometry, number theory, and solid geometry. Many of the theorems, such as those related to triangles, circles, and various geometric figures, are still taught in schools today.

3. **Logical Deduction**: Euclid's work emphasized the importance of logical reasoning and structured proof, which are fundamental practices in modern mathematics.

4. **Influence on Later Mathematicians**: His work influenced countless mathematicians throughout history, including generations of scholars in the Islamic Golden Age, Renaissance Europe, and beyond. The structure and style of "Elements" became a model for mathematical textbooks.

Many of Euclid's concepts, definitions, and proofs continue to form the foundation of geometry courses around the world, making his contributions evergreen in the field of mathematics.