Beseler has used knobs in several different sizes. These replacement discs are approximately 7/8" (22mm) in diameter to fit the larger size knobs on certain Beseler colorheads, such as the 23C Dual Dichro, and some Beseler color analyzers, such as the PM2. Smaller 3/4" (19mm) size discs are also available, please see the separate listing. Sold as a set of three.