About nesting and precomposing

Nesting is the inclusion of one composition within another. The nested composition appears as a layer in the containing composition.

If you want to group some layers that are already in a composition, you can precompose those layers. Precomposed layers are placed in a new nested composition, and the new nested composition becomes the source for a single layer in the original composition. The new composition appears in the Project panel and is available for rendering or use in any other composition. You can nest compositions by adding an existing composition to another composition, just as you would add any other footage item to a composition.

Nested compositions are sometimes called precompositions. When a precomposition is used as the source footage item for a layer, the layer is referred to as a precomposition layer.

During rendering, the image data and other information can be said to flow from each nested composition into the composition that contains it. For this reason, nested compositions are sometimes referred to as being upstream of the compositions that contain them, and the containing compositions are said to be downstream of the nested compositions that they contain. A set of compositions connected through nesting is called a composition network. You can navigate within a composition network using the Composition Navigator and Mini-Flowchart. (See Opening and navigating nested compositions.)

Precomposing and nesting are useful for managing and organizing complex compositions. By precomposing and nesting, you can do the following:

Apply complex changes to an entire composition: You can create a composition that contains multiple layers, nest the composition within the overall composition, and animate and apply effects to the nested composition so that all of the layers change in the same ways over the same time period.
Reuse anything you build: You can build an animation in its own composition and then drag that composition into other compositions as many times as you want.
Update in one step: When you make changes to a nested composition, those changes affect every composition in which it is used, just like changes made to a source footage item affect every composition in which it is used.
Alter the default rendering order of a layer: You can specify that After Effects render a transformation (such as rotation) before rendering effects, so that the effect applies to the rotated footage.
Add another set of transform properties to a layer: The layer that represents the composition has its own properties, in addition to the properties of the layers that it contains. This allows you to apply an additional set of transformations to a layer or set of layers.

For example, you can use nesting to make a planet both rotate and revolve (moving like the Earth, which spins on its own axis and also travels around the Sun). To create such a system, animate the Rotation property of the planet layer, precompose that layer, modify the Anchor Point property of the precomposition layer, and then animate the Rotation property of the precomposition layer.

Preferences and composition settings that affect nested compositions

Because a precomposition is itself a layer, you can control its behavior using layer switches and composition switches in the Timeline panel. You can choose whether changes made to the switches in the containing composition are propagated to the nested composition. To prevent layer switches from affecting nested compositions, choose Edit > Preferences > General (Windows) or After Effects > Preferences > General (Mac OS), and then deselect Switches Affect Nested Comps.

In the Advanced tab of the Composition Settings dialog box (Composition > Composition Settings), choose Preserve Resolution When Nested or Preserve Frame Rate When Nested Or In Render Queue for a composition to retain its own resolution or frame rate, and not inherit those settings from the containing composition. For example, if you deliberately used a low frame rate in a composition to create a jerky, hand-animated result, you should preserve the frame rate for that composition when it is nested. Similarly, the results of rotoscoping may look wrong when converted to a different frame rate or resolution. Use this setting instead of the Posterize Time effect, which is less efficient.

Changing the current time in one panel updates the current time in other panels associated with that composition. By default, the current time is also updated for all compositions related to the current composition by nesting. To prevent compositions related by nesting from updating their current times when you change the current time in one composition, deselect the Synchronize Time Of All Related Items preference (Edit > Preferences > General (Windows) or After Effects > Preferences > General (Mac OS)).