user: Describe Face from the Blender manual or Python API, including usage steps and snippets.

assistant: ### Basic Concepts

#### Skin Bone Chains

[[image reference removed]](../../../_images/addons_rigging_rigify_rig-features_face-chains.png)

The foundation of the Rigify face is a network of Bendy Bone [chains](rig_types/skin.html#rigify-rigs-skin-basic-chain) with
controls placed at every bone end. These controls affect all bones that meet at that specific point.

When the controls are merely translated, the B-Bone chains retain the normal automatic bezier handle behavior.
Local rotation and/or scaling of the controls are applied on top of that.

In case of [certain chains](rig_types/skin.html#rigify-rigs-skin-stretchy-chain), the transformation of the end and/or middle
controls is interpolated to other controls located between them. In such cases the controls often have different
colors and/or shapes.

Additionally, certain controls have [arbitrary constraints](rig_types/skin.html#rigify-rigs-skin-glue) that partially copy
transformation from nearby control points.

#### Specialized Controllers

Certain areas of the face, like eyes or mouth, have additional specialized controllers that apply custom behavior
on top of the chains and their controllers within the relevant area.

### Eyes

[[image reference removed]](../../../_images/addons_rigging_rigify_rig-features_eye-controls.png)

The [eyes](rig_types/face.html#rigify-rigs-face-skin-eye) have the following controls in addition to the eyelid chains:

Master
:   This large circular control can be used to transform the whole eye as one unit.

Common Target
:   This large control enveloping all individual eye targets has a switchable parent and can
    be used to specify the point that the eyes should look at.

Eye Target
:   These small circle controls within the common target control specify the point targeted by each
    individual eye. Their local scale can also be used to affect the iris or pupil of the eye,
    depending on how it was weight painted.

The rig properties panel contains the following options:

[[image reference removed]](../../../_images/addons_rigging_rigify_rig-features_eye-properties.png)

Eyelids Follow Slider
:   Controls how much the rotation of the eyeball affects the eyelids. Depending on the sub-rig generation
    options, this slider can be split to separately control the horizontal and vertical directions.

Eyelids Attached Slider Optional
:   If enabled in the sub-rig generation options, this slider can be used to disable the mechanism that
    forces the eyelids to conform to the sphere of the eye.

Parent Parent Switch
:   Selects the parent for the common target control.

### Mouth

[[image reference removed]](../../../_images/addons_rigging_rigify_rig-features_mouth-controls.png)

The [mouth](rig_types/face.html#rigify-rigs-face-skin-jaw) has the following controls:

Jaw Master
:   Controls rotation of the jaw, directly affecting the main jaw deform bone, as well
    as chains fully belonging to the jaw. Chains forming the lip loop(s) are adjusted to
    open the mouth as the jaw rotates or moves.

Mouth Master
:   This control uniformly transforms the lips without moving the jaw.

The rig properties panel contains the following options:

[[image reference removed]](../../../_images/addons_rigging_rigify_rig-features_mouth-properties.png)

Mouth Lock Slider
:   This slider can be changed from 0 to 1 in order to suppress opening of the mouth
    when the jaw rotates or moves.