For a double-rod closet, the standard height of the top rod is 81.75 inches from the floor, and the standard height of the bottom rod is 40.5 inches. For a single-rod closet, the standard rod height is 66.5 inches.
Double-rod closets are intended to be used for hanging shirts and pants, which are generally less than 40 inches long. Since these items of clothing are shorter, two layers of clothing can fit into a standard 84-inch high closet space, thus the double rods.
Single-rod closets are intended to be used for hanging coats and dresses. Taller individuals may prefer a rod that is placed higher than normal to keep long articles of clothing from touching the floor.