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.

ER32 series collets have a capacity of 1mm-20mm. Each ER32 collet has a range of (1 mm). The size indicated on the collet is the largest size it can hold and can be collapsed smaller within its collapse range of (0.9mm). For example, ER32-6mm can grab a round shank from 6mm diameter to 5.1mm diameter.

ER collets have a wide clamping range and high accuracy. While they are designed for tool holding, they work great for work holding in small lathes. With a full set of ER collets you can hold any diameter that is in the clamping range of the set. There are several types of collets that look like ER collets.

Off-the-shelf, hardened and ground collets are more accurate and provide longer lasting concentricity than the hard or soft jaws of a standard jaw chuck. Collets are also better for gripping extrusions or special shapes than three equally spaced chuck jaws.