Torsion springs are designed and wound to be actuated rotationally, and to provide an angular return force. They are used in all types of hinges and assemblies such as self-closing doors and lids. There are often many options for leg configuration so the spring can be attached in different ways.  The most important size parameters to consider when searching for torsion springs include design units and dimensions.  Design units can be specified in English or metric units.  Some manufacturers may specify both.  Important dimensions include inner diameter, outer diameter, wire diameter, and spring length.  The inner diameter is typically specified when the spring is required to slip over a mandrel with sufficient clearance to operate freely.  The outer diameter is typically specified when the spring is required to fit into a circular hole with sufficient exterior clearance to operate freely, or if there are outer housing clearance issues.  The wire diameter is the diameter of the actual spring wire.  The spring length is the length of the spring coil.