Prism

Prism, in Geometry, a solid figure which can be most easily conceived of if we imagine a number of plane figures (triangles, quadrilaterals, &c.) exactly similar in form and size to be cut out of paper or any thin plate, and piled one above the other, and then the whole pile to become one body. It will thus be seen that the top and bottom of the prism are similar, equal, and parallel to each other, and that the sides are plane figures, rectangular if the prism be 'right' (i.e. if in the above illustration the pile of plane figures be built up perpendicularly), and rhomboidal if the prism be 'oblique' (i.e. if the pile slope to one side); but under all circumstances the sides of a prism must be parallelograms. The top and bottom faces may be either triangles, squares, parallelograms, or quadrilaterals of any sort, or figures of five, six, seven, &c. sides, provided only both are alike; and the number of sides in the plane figure which forms the top or bottom of course determines the number of faces of the prism; thus, in a triangular prism, there are five faces in all (three sides and two ends); in a quadrangular prism, six faces (four sides and two ends), &c. If two prisms, one being 'right,' and the other 'oblique,' have their bases of equal area, and be of the same vertical height, their solid content is the same, and is found by multiplying the area of the base by the vertical height. The parallelopiped is a quadrangular prism, and the cube is a particular case of the parallelopiped.