This part of ISO 3601 specifies dimensions and tolerances for five types of anti-extrusion rings, which are also known as back-up rings:
a) spiral type (T1);
b) angle cut type (T2);
c) solid type (T3);
d) angle cut concave type (T4);
e) solid concave type (T5).
These anti-extrusion rings are for use with selected O-ring sizes as specified in ISO 3601-1 and the relevant housing dimensions specified in ISO 3601-2.