The standard series are: ER-8, ER-11, ER-16, ER-20, ER-25, ER-32, ER-40, and ER-50. The "ER" name came from an existing "E" collet (which were a letter series of names) which Rego-Fix modified and appended "R" for "Rego-Fix". The series number is the opening diameter of the tapered receptacle, in millimetres.

The numbers represent the opening diameter of the tapered receptacle in millimeters. As you tighten the nut on your holder, the ER collet collapses together to hold round tools or stock from 0.5-2 mm (or 0.0197-0.0787 in., depending on collet size) smaller than the collet internal diameter.

This is the 'standard' tool holder for small routers. ER is the 'name' 11 means that it has an 11 mm diameter at the bottom of the taper in the holder. Why is a collet system is used, why not just mount a chuck which would be more versatile and easier to adjust? A Collet system will: Clamp the bit more firmly.