Pearl beads: Button Pearls are drilled through the short center dimension of the pearl. The sides of the pearl tend to be a bit flatter than a potato pearl. These pearls are sometimes also called "side-drilled pearls" or "rondelle pearls" at Topearl jewelry store.
Button Pearls only are measured the diameter of drilled side, so the pearl size is given a single number (e.g. 6mm) or a small range (6-6.5mm).