Black Lucite Piano Caster Cups Set of 4 for Upright Pianos Black Lucite Piano Caster Cups - Set of 4 4-1/2" Outside Diameter, 2-3/4" Inside Diameter Lucite is a strong acrylic product, that is practically unbreakable Designed to be placed under a piano's legs in order to protect floors and carpets Ideal for upright pianos