16. To find the lateral surface of a prism: Rule - Multiply the perimeter of the base by the altitude.

17. To find the lateral surface of a regular pyramid: Rule - Multiply the perimeter of the base by one-half the slant height. Fig. 9.

18. To find the lateral surface of the frustrum of a regular pyramid: Rule - Multiply the perimeters of the two ends by one-half the slant height. Fig. 10.