manual/include/handling-overlapping-notes.html

<p>
  Every MIDI note consists of two messages, a NoteOn and a NoteOff. Each one
  has a note number and a channel (also a velocity, but that isn't relevant
  here). The MIDI standard stresses that it is invalid to send a second NoteOn
  for the same note number on the same channel before a NoteOff for the first
  NoteOn. It is more or less impossible to do this with a physical MIDI
  controller such as a keyboard, but remarkably easy to trigger when editing
  in a DAW&mdash;simply overlapping two instances of the same note will do it.
</p>
<p>
  Ardour offers many options for how to deal with instances where you overlap
  two instances of the same note. Which one to use is a per-session property
  and can be modified from <kbd class="menu">Session &gt; Properties &gt; Misc &gt; MIDI
  Options</kbd>.
</p>

<dl class="wide-table">
  <dt>never allow them</dt>
  <dd>Edits that would create note overlaps are not allowed</dd>
  <dt>don't do anything in particular</dt>
  <dd>Ardour leaves overlapping notes alone&mdash;the behaviour of a MIDI receiver (plugin or hardware) is undefined</dd>
  <dt>replace any overlapped existing note</dt>
  <dd>When one note is moved to overlap another, remove the one that wasn't being moved</dd>
  <dt>shorten the overlapped existing note</dt>
  <dd>When one note is moved to overlap another, shorten the one that wasn't moved so that there is no overlap</dd>
  <dt>shorten the overlapping new note</dt>
  <dd>When one note is moved to overlap another, shorten the one that was moved so that there is no overlap</dd>
  <dt>replace both overlapping notes with a single note</dt>
  <dd>When one note is moved to overlap another, merge them both to form one (longer) note</dd>
</dl>
<p>
  Changing the option in use will not retroactively make changes&mdash;it will
  only affect new note overlaps created while the option remains chosen.
</p>
<p class="warning">
  Ardour does not check for note overlaps across tracks or even across regions.
  If you create these, it is your responsibility to deal with the consequences.
</p>