A rectangle is a four-sided polygon having two sets of parallel, congruent sides and four interior angles of 90 degrees each. If you have a form that fits that criteria, it is also all of these things: a planar figure. A rectangle can be any size, but the most common sizes are 30 by 40 inches (75 by 100 cm), which is often called a standard sheet of paper, and 16 by 32 inches (40 by 80 cm), which is called a standard office desk pad. There are many ways to describe the properties of a rectangle. The best way is to use mathematics to prove that it is indeed a rectangle. There are several methods used in geometry to prove that certain figures are equivalent to one another or not. In this case, we will use the fact that rectangles are the only figures that have two pairs of opposite angles that are equal. By using this property along with some simple math, we can conclude that whatever shape has these same angle pairs must be a rectangle. Here are the two pairs of angles that define a rectangle: Two opposite angles are equal. And two other opposite angles are equal. This means that if you were to draw lines from each corner of this figure to any point on its surface, those lines would always cross at right angles, or 90 degrees, away from each other. These are very important properties for a rectangle to have.