To create a compo from several Layers, select them and press 
                      
                        [Ctrl] + [Shift] + [C]
                      
                    
A compo is a collection of 
                    2D Layer
                    A single 2D Layer
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                    , composited together.
                    A compo can contain 
                    2D Layer
                    A single 2D Layer
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                     and 
                    3D Layer
                    A single 3D Layer (3D object, particle system, ect)
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                     together. It can also contain 
                    Tool
                    An element that adds logic and/or functionality to your compo, that is not visible on screen
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                     and 
                    Audio File
                    Sound track in files
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                    .
                    A compo can also exist as a layer (pre-comp). That means layers can be grouped inside a compo, inside your main compo.
                    A compo has a main animation, that can either be a 
                    Timeline
                    
        Timelines allow for easy playback, cutting, and editing of video layers, as well as animation of layers and their parameters, editable with the Timeline Editor.
     
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                     or a 
                    Cue List
                    Add cues one after another to encode your Show
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                    . A compo with a main timeline can be played back like any other animated media (video, animated generative layer, etc).
                    A compo has a main 
                    Camera
                    A camera filming 3D content
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                    . It defines the point of view that will be used to show the 
                    3D Layer
                    A single 3D Layer (3D object, particle system, ect)
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                     in the compo.
                    When saved on disk, a compo has the extension ".compo".
                    You can also 
                    Bake compositions
                    
      How to transform a Compo into a video file inside a composition to gain fps
    
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                     and 
                    Export and Consolidate
                    
      How to collect all files of a composition in order to share it
    
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                     them (except on Synth).
                    To save a compo. Simply press 
                    
                      [Ctrl] + [S]
                     or do 
Menu for Opening, Saving and Closing operations
                    -> "Save/Save as"
                    
