From BlenderWiki

Armature Structure

As said, armatures mimic real skeletons. They are made out of bones, which are (by default) rigid elements. But you have more possibilities than with real skeletons… In addition to the “natural” rotation of bones, you can also translate and even scale them! And your bones do not have to be connected to each other, they can be completely free if you want. However, the most natural and useful setups imply that some bones are related to each others, forming so-called “chains of bones”, some sort of “limbs” in your armature, as detailed below.

Chains of Bones

The bones inside an armature can be completely independent from each other (i.e. the modification of one do not affect the others)… but this is not often a useful setup! To create something like a leg, you want that all bones “after” the thigh one move “with” it, as if they were parented. Well, this is exactly what happens in armatures – by “parenting” bones to each other, you create such “limbs”, called in Blender “chains of bones”. These chains can of course be ramified, e.g. for the five fingers attached to a single “hand” bone…

Bones are chained by linking the tip of the parent to the root of the child. Root and tip can be connected, i.e. they are always exactly at the same point, or they can be free, like in a standard parent relationship.

A same bone can be the parent of several children, and hence be part of several chains at the same time.

The bone at the beginning of a chain is called its root bone, and the last bone of a chain is the tip bone (don’t confuse them with bones’ ends names…).

Chains of bones are a particularly important topic in posing (especially with the standard forward kinematics versus “automatic” inverse kinematics posing techniques…). You create/edit them in Edit mode, but except for connected bones, their relationships have no effect on bone transformations in this mode (i.e. transforming a parent bone won’t affect its children…).

Editing Bones Relationships

This is detailed in the editing pages, but let’s have here a quick look at this important feature.

The easiest way to manage bones relationships is to use the Relations panel Bone context:

• First, select the bones you want to edit (order does not matter here).
• To parent a bone to another one, select the name of this parent in its drop-down Parent list.
• To unparent a bone, just select the void entry in the same Parent list.
• To connect a bone to its parent, enable its small Con button.
• To unconnect a bone, disable its Con button.