**Concave**describes shapes that curve inward, like an hourglass.**Convex**describes shapes that curve outward, like a football (or a rugby ball).

If you stand in front of a *concave* mirror, your reflection will look taller. If you stand in front of a *convex* mirror, the opposite will happen—your reflection will appear shorter. Does this help you understand the difference between *concave* and *convex*?

## Definition of *Concave*

*Concave* describes shapes that curve inward. The inside part of a bowl is a *concave* shape.

*Concave* can also be used as a noun. A concave is a surface or a line that is curved inward. In geometry, it is a polygon with at least one interior angle greater than 180°.

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## Definition of *Convex*

A *convex* shape is the opposite of a *concave* shape. It curves outward, and its middle is thicker than its edges. If you take a football or a rugby ball and place it as if you’re about to kick it, you’ll see that it has a convex shape—its ends are pointy, and it has a thick middle.

Just like *concave*, *convex* can be used as a noun for a surface or line that curves outward, and it also has a use in geometry, where it describes a polygon with interior angles less than or equal to 180°.

## How to Remember the Difference Between *Concave* and *Convex*

Finding a mnemonic device for *concave* is easy enough. There’s a “cave” in *concave*, and caves are inward curvatures. That doesn’t work as well for *convex*, but if you can recall what we said for *concave*, all you need to do is remember that *convex* is the exact opposite.