# Introduction to trees

Continuing with our introduction to combinatorics, this time we explored the structure side of things and introduced graph theory.

A graph is a diagram with dots called vertices and lines joining the dots called edges. A tree is a graph that doesnâ€™t have any loops (also called cycles). How many trees are there of a given size? How can you communicate the structure of a tree efficiently without drawing a picture?

We investigated these questions and more.