Non-Linear Workflow Principles
The Cue window allows working with music ideas in a non-linear fashion. Instead of navigating the timeline and placing regions of audio and MIDI data at a particular point in time, you deal with short clips that contain rhythmic and melodic patterns and can be triggered to play a certain amount of times, then automatically trigger another clip to be played.
The concept has been introduced and popularized by Ableton and since then found its way into many other applications. Ardour draws many ideas from Ableton Live, as well as from several other digital audio workstations, and adapts them to Ardour's specifics. If you are familiar with Live, you will find many aspects familiar, but you should not expect the Cue's feature set to be a 100% copy of that from any other application.
Here are some basics concepts of the non-linear workflow shared by multiple applications including Ardour.
Grid and scenes
All clips are organized in a kind of a grid. The grid provides an overview of all the musical ideas, all the rhythmic patterns, short melodies, and sound effects that you can use in a composition.
One dimension of the grid, usually represented by a track, would accumulate clips played with roughly the same kind of an instrument, e.g. all drum patterns, or all basslines etc.
The other dimension, usually called scenes (or cues, in Ardour) would organize these clips so that you would be able to play multiple clips at the same time by pressing just one button. So if you want a particular bassline played along a particular drum sequence, you would place them in the same scene.
Ardour specifics are explained in the Cue window elements chapter.
Slots and clips
Cells in a grid are usually called slots. They are a kind of a container that can hold an audio or a MIDI clip. Typically, a clip can be loaded into a slot from a disk by pointing the file selector to it, or loaded from a pre-recorded library of reusable clips, or recorded in place. You will find more information about that in the Populating the cue grid chapter.
Launching
In a non-linear workflow, a clip can be triggered to play in multiple ways. Most of the time it's either pressing a corresponding silicon pad on an external grid controller attached via MIDI, or scrolling the mouse wheel downwards over the slot that contains the clip, or just clicking a 'Play' button next to clip's name.
Usually you can configure a slot to respond to some ways to trigger clip playback and ignore others. We'll talk about it in the Clip Launch Options chapter.
Follow actions
A clip can play in a loop until you stop it directly, or it can play a user-defined amount of time and the trigger another clip in the track. Say, you start a composition with one rhythmic pattern played four times and you want the next rhythmic patterns to play eight times, then move to a third one.
This is typically achieved through so called follow actions. In an example above, for the first clip (or, rather, slot) you can set a follow count (4 times), and use the follow action usually called "Next". This will get the clip in that first slot to play 4 times then trigger the playback of a clip in the second slot.
Every application has its own set of follow actions. Most common ones are repeating the clip indefinitely, triggering the previous/next slot, or jumping to a slot in a particular scene.
You can read more about follow actions in Ardour here.
Musical time and stretching
In a non-linear workflow, all work is happening in musical time: both audio and MIDI clips are measured in bars and beats.
By default, an application that supports a non-linear workflow will attempt to estimate beats per minute in an audio clip and then stretch or squeeze the clip so that it would match the bpm of the session and wrap neatly around bars. That way, a clip that originally has a different tempo that the one in the session would stay in sync with other clips.
Stretch options in Ardour are explained here.