Ardour's MIDI editing is based on two basic principles:

1. Editing should be done without having to enter a new window
2. Editing should be able to carried out completely with the keyboard, or completely with the mouse, or with any combination of the two.

Currently, MIDI editing is primarily restricted to note data. Other kinds of data (controller events, sysex data) are present and can be added and deleted, but not actually edited.

Fundamentals of MIDI Editing in Ardour 3

MIDI, just like audio, exists in regions. MIDI regions behave like audio regions: they can be moved, trimmed, copied (cloned), or deleted. Ardour allows either editing MIDI (or audio) regions, or MIDI region content (the notes), but never both at the same time. The e key (by default) toggles between region level and note level editing, as will double-clicking on a MIDI region.

One very important thing to note: editing note information in Ardour occurs in only a single region. There is no way currently to edit in note data for multiple regions at the same time, so for example you cannot select notes in several regions and then delete them all, nor can you copy-and-paste notes from one region to another. You can, of course, copy and paste the region(s), just as with audio.