Belllampshades curve from the top to the bottom. This can be a gentle curve, or an exaggerated one. Bells come in many shapes, from a cylinder to a square. The most popular bell has a round top and bottom, with a smaller top. The curve of the bell lends itself to a graceful look, which works well on many traditional lamps and often on Asian inspired styles.
If you are replacing an existing lampshade it may help to measure it first. Start by measuring the widest part (diameter) of the top and bottom of your shade, then finish by measuring the slant/height along the side.