Read this chapter, which summarizes all properties of various quadrilaterals, including the properties of their diagonals.

The prefix "quad-" means "four", and "lateral" is derived from the Latin word for "side". So a quadrilateral is a four-sided polygon. Since it is a polygon, you know that it is a two-dimensional figure made up of straight sides. A quadrilateral also has four angles formed by its four sides.

\begin{align*}AB\end{align*}\begin{align*}BC\end{align*}\begin{align*}CD\end{align*} and \begin{align*}DA\end{align*} are the sides and \begin{align*}A\end{align*}\begin{align*}B\end{align*}\begin{align*}C\end{align*} and \begin{align*}D\end{align*} are the vertices of the quadrilaterals.

Line segments \begin{align*}AC\end{align*} and \begin{align*}BD\end{align*} joining two non-consecutive vertices are called diagonals.

Two sides like \begin{align*}AB\end{align*} and \begin{align*}AD\end{align*} having a common endpoint are called adjacent sides.

There are many common special quadrilaterals that you should be familiar with. Below, these special quadrilaterals are described with their definitions and some properties.