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.

Compared with standard triple-jaw chucks with soft jaws, standard collets tend to be inexpensive and more accurate for gripping in the small-to-medium range. Conversely, three-jaw chucks are usually ineffective and cumbersome when used to grip items this small.

BT40 toolholder has a slightly shorter taper by about 4mm. This effectively reduces the contact area in the spindle and means BT uses longer studs. So two different studs for CAT40 and BT40 are needed. 2. The style and thickness of the flange is different.

Collets are used to hold a tool in a central position as it rotates. As the tool is inserted into the collet, the device expands and provides uniform pressure that holds the tool in place. However, if the collet stretches too far, it cannot secure the instrument properly and may lose shape.