Just as the rectangle is the chief surface considered in arithmetic, so
the rectangular solid is the chief solid body.
A rectangular solid is bounded by six rectangular surfaces, each
opposite pair of rectangles being equal and parallel to each other.
A rectangular solid thus has three dimensions--length, breadth, and
thickness.
If the length, breadth, and thickness are all equal to one another, the
solid is called a cube. Hence, a cubic foot, the unit of volume, is a
solid body whose length, breadth, and thickness are each a linear foot.
Similarly, a cubic inch measures one linear inch in length, breadth, and
thickness; and a cubic yard measures one linear yard in length, breadth,
and thickness.
[Illustration: One cubic foot]
The number of cubic feet (or inches, or yards) in the volume of a
rectangular solid is equal to the number of linear feet (or inches, or
yards) in the length, multiplied by the number of linear feet (or
inches, or yards) in the breadth, multiplied by the number of linear
feet (or inches, or yards) in the thickness.