====== HingeJoint ======

  * ''HingeJoint'' does not constrain the absolute ''rotation'' or ''localRotation'' of the Transform that it is attached to. It constrains the **relative** rotation between its own RigidBody and the Rigidbody in the ''ConnectedBody'' field.

  * ''HingeJoint.angle'' and ''HingJoint.axis'' do not represent the rotation of the Transform that it is attached to. Ex:<code csharp>Quaternion.AngleAxis(hinge.angle, hinge.axis) 
// is *not* equivalent to either of
hinge.transform.rotation
hinge.transform.localRotation
</code> 

=== Rest Angles ===
To quote the [[https://docs.unity3d.com/ScriptReference/HingeJoint-angle.html|official docs]]:

> The rest angle between the bodies is always zero at the beginning of the simulation.

The consequence of this is that ''HingeJoint.angle'' does not depend on the starting orientation of the joint's ''RigidBody'' or the connected ''RigidBody''. And because ''HingeJoint.limits'' represents limits on ''HingeJoint.angle'', the limits are also independent of any starting orientation of the bodies. 

An example: Consider a ''GameObject'' with rotation ''(0, 0, 0)'' and a ''HingeJoint'' on the x-axis with limits of ''[-90, 90]''. When played as-is, this ''GameObject'' will be free to rotate between ''(-90, 0, 0)'' and ''(90, 0, 0)''. However, if you rotate this ''GameObject'' in the editor such that it starts with rotation ''(90, 0, 0)'' and play it, the ''GameObject'' will then be free to rotate between ''(0, 0, 0)'' and ''(180, 0, 0)'' - even though the angle limits have not changed.

=== HingeJoints with moving parents: ===

In most cases, ''HingeJoint'' should have a ''ConnectedBody'' assigned. If left empty it causes your ''HingeJoint'' to be anchored to the global physics coordinate system. If your ''HingeJoint'' is the child of an object that moves, the joint will apply its limits within the global physics coordinate system - not in the joint's local coordinate system.

To ensure that your ''HingeJoint'' behaves nicely, create a ''RigidBody'' on the parent object and set that as the ''ConnectedBody''.((The parent body can have ''IsKinematic'' enabled if you don't want it to act like a physics object.)) In this configuration, the ''axis'' and ''angle'' of the ''HingeJoint'' will correspond to the ''localRotation'' of the ''Transform'' ((though it will still be relative to any initial ''localRotation'')). This makes the joint behavior much easier to reason about!