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# ARSkeletonDefinition

The hierarchy of joints and their names.

```
class ARSkeletonDefinition
```

## Overview

A skeleton definition establishes the relationship of joints that make up a 3D or 2D body’s [`skeleton`](/documentation/ARKit/ARBody2D/skeleton), in which joints connect to other joints to compose a single skeleton in a parent-child hierarchy. Use [`parentIndices`](/documentation/ARKit/ARSkeletonDefinition/parentIndices-82iw2) to identify the hierarchy for a given skeleton definition.

ARKit names particular joints that are crucial to body tracking. You can access named joints by calling [`indexForJointName:`](/documentation/ARKit/ARSkeletonDefinition/indexForJointName:) and passing in one of the available [`jointNames`](/documentation/ARKit/ARSkeletonDefinition/jointNames) identifiers.

## Topics

### Locating in the Physical Environment

[`neutralBodySkeleton3D`](/documentation/ARKit/ARSkeletonDefinition/neutralBodySkeleton3D)

The 3D skeleton in neutral pose.

[`defaultBody3DSkeletonDefinition`](/documentation/ARKit/ARSkeletonDefinition/defaultBody3D)

The default skeleton definition for bodies defined in 3D.

### Locating in Screen Space

[`defaultBody2DSkeletonDefinition`](/documentation/ARKit/ARSkeletonDefinition/defaultBody2D)

The default skeleton definition for bodies defined in 2D.

### Getting Joint Information

[`jointNames`](/documentation/ARKit/ARSkeletonDefinition/jointNames)

A collection of unique joint names.

[`jointCount`](/documentation/ARKit/ARSkeletonDefinition/jointCount)

The skeleton’s total number of joints.

[`index(for:)`](/documentation/ARKit/ARSkeletonDefinition/index(for:))

Returns the index for a given joint identifier.

[`parentIndices`](/documentation/ARKit/ARSkeletonDefinition/parentIndices-u2u9)

The parent index for each joint.

[`-  indexForJointName:`](/documentation/ARKit/ARSkeletonDefinition/indexForJointName:)

Returns the index for a given joint identifier.

[`parentIndices`](/documentation/ARKit/ARSkeletonDefinition/parentIndices-82iw2)

The parent index for each joint.

## Relationships

### Conforms To

[`CustomStringConvertible`](/documentation/Swift/CustomStringConvertible)

[`CVarArg`](/documentation/Swift/CVarArg)

[`CustomDebugStringConvertible`](/documentation/Swift/CustomDebugStringConvertible)

[`Equatable`](/documentation/Swift/Equatable)

[`Hashable`](/documentation/Swift/Hashable)

[`Sendable`](/documentation/Swift/Sendable)

[`NSObjectProtocol`](/documentation/ObjectiveC/NSObjectProtocol)

[`SendableMetatype`](/documentation/Swift/SendableMetatype)

### Inherits From

[`NSObject-swift.class`](/documentation/ObjectiveC/NSObject-swift.class)

---

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