# Definition:Base of Solid Figure

## Definition

The **base** of a solid figure is one of its faces which has been distinguished from the others in some way.

The solid figure is usually oriented so that the **base** is situated at the bottom.

### Base of Parallelepiped

One of the faces of the parallelepiped is chosen arbitrarily, distinguished from the others and called a **base of the parallelepiped**.

The opposite face to that face is also referred to as one of the **bases**.

It is usual to choose one of the **bases** to be the one which is conceptually on the bottom.

In the above, $ABCD$ and $EFGH$ would conventionally be identified as being the **bases**.

### Base of Pyramid

The polygon of a pyramid to whose vertices the apex is joined is called the **base** of the pyramid.

In the above diagram, $ABCDE$ is the **base** of the pyramid $ABCDEQ$.

### Base of Prism

The **bases** of a prism are the two parallel polygons which form the faces at either end of the prism.

In the above diagram, the faces $ABCDE$ and $FGHIJ$ are the **bases** of the prism.

### Base of Cone

The plane figure $PQR$ is called the **base** of the cone.

#### Base of Right Circular Cone

Let $\triangle AOB$ be a right-angled triangle such that $\angle AOB$ is the right angle.

Let $K$ be the right circular cone formed by the rotation of $\triangle AOB$ around $OB$.

Let $BC$ be the circle described by $B$.

The **base** of $K$ is the plane surface enclosed by the circle $BC$.

In the words of Euclid:

*And the***base**is the circle described by the straight line which is carried round.

(*The Elements*: Book $\text{XI}$: Definition $20$)

### Base of Cylinder

In the words of Euclid:

*And the***bases**are the circles described by the two sides opposite to one another which are carried round.

(*The Elements*: Book $\text{XI}$: Definition $23$)

In the above diagram, the **bases** of the cylinder $ACBEDF$ are the faces $ABC$ and $DEF$.