Top Diameter 2. Bottom Diameter 3. Side length Barrel shaped shades have top diameter only an inch or two smaller than bottom diameter.
Flared shades have a top diameter significantly smaller than bottom diameter.
One need not replace an old barrel shade with another. Update your lighting in one fell swoop and order a botanical shade. Please do not send big barrel shades. Clips directly on to bulb, maximum 60 watts recommended. This is a specialty shade that threads onto the socket and holds the shade at a slight angle, like a bridge lamp.