Devolution™ baseplates allow you to use Chris King headsets designed for larger diameter steerer tubes on forks with smaller diameter steerer tubes. Devolution™ baseplates are NOT compatible with DropSet™ headsets. They are only compatible with press in cup headsets. The first size indicates the bearing size, and the second size is the steerer tube size. PHS450 1-1/8" to 1" PHS433 1-1/4" to 1-1/8" PHS626 1-1/2" to 1-1/8" PHS629 1-1/2" to 1-1/4"