All rods require pole dividers spaced at least 24 inches apart to give enough room to slide the hangers around. Any span wider than 34 inches will cause a sagging rod.
A tall-level single rod requires a height of 60 to 65 inches from the floor and is intended for hanging dresses, long coats and the clothes of tall people. A height of 65 inches is recommended to keep longer clothes off the floor.
Medium height single rods should be hung at about 45 inches from the floor. This level is meant for hanging men's and women's suits, women's blouses and skirts and men's pants and shirts. This is also a good height for the clothes of 6 to 12 year olds.
Short or low level single rods work for shirts and blouses. They are also usable for clothes of 3 to 5 year olds.
Double rods are generally used on the men's side of the closet. The upper rod is hung at a level of about 80 inches with the lower rod at a 39-inch height. This setup allows room for most types of clothing --- shirts and blouses, jackets and pants folded over a hanger, for instance.
Top shelves generally run the entire length of the closet. They should be 11 to 15 inches deep and should be spaced at least 3 inches above the hanger rod.