# What is a Quadrilateral?

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A **quadrilateral** is a geometric figure formed by joining four points, any three of which are non-collinear, in some order. In other words, a quadrilateral is any *closed, four-sided figure*. Two examples are shown below:

We note that a quadrilateral has four sides, four angles, and four vertices. When we have to specify a quadrilateral, we do it keeping in mind the order of the vertices. For example, consider the following quadrilateral:

We could name this quadrilateral ABCD, BCDA, ADCB, DCBA, etc, but we *cannot* name it ACBD, BACD, etc., since in the latter names, the order of the vertices is wrong.

A quadrilateral ABCD will have two **diagonals:** AC and BD.