# Section types

Sections are classified as *skill,* *lesson,* or *theorem.* Every section should begin with a summary of what you're going to learn and what you must already know. What follows is a description of each section type.

## Skill

A "skill" is something that can be taught and practiced. Therefore, it must have a "lesson" and "practice" section.

## Lesson

A "lesson" can exist without being part of a "skill" For example, we could have a lesson on the history of squaring the circle. It's just some interesting stuff with nothing to practice.

## Theorem

A "theorem" consists of the subsections *Intro,* *Proof,* and *Using the theorem.* The *Intro* describes what the theorem tells us, the *Proof* tells us why the theorem is true, and *Using the theorem* gives us practice in applying the theorem to solve math problems.