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Edit notes in the Piano Roll Editor in GarageBand on Mac
You need to select notes before you can edit them. You can select notes by clicking, Shift-clicking, dragging to make a marquee selection, and using the keyboard at the left edge of the Piano Roll Editor.
The Piano Roll Editor header shows the number of selected notes and the region name. When multiple overlapping notes are selected, the Piano Roll Editor header shows the chord name. When no notes are selected, the header shows the name of the most recently selected MIDI region, or the number of regions selected.