The Piano Roll Editor shows notes in MIDI regions as rectangles on a time grid. The position of each note on the grid shows the time position where it starts playing, its duration (length), and pitch. Note velocity is indicated by color.
You can edit individual notes by moving them, resizing them, dragging them vertically to change their pitch, and in a variety of other ways.
Piano Roll Editor menu bar: Contains menus with region editing commands and functions, as well as buttons for selecting editing tools and zooming the editor.
Piano Roll Editor inspector: Contains controls for quantizing note timing and adjusting note pitch and velocity.
Display area: Shows the notes in the MIDI region or regions as bars on a time grid.
Ruler: Shows time divisions so you can align and edit notes at precise time positions.
Playhead: Reflects the current playback position.
Do one of the following:
Double-click a MIDI region in the Tracks area.
You can change this default behavior for opening the Piano Roll Editor in Logic Pro > Preferences > General.
Select a software instrument or external instrument track, then click the Editors button 
in the control bar.
Select a software instrument or external instrument track, then choose View > Show Editors.
Choose Window > Open Piano Roll (or press Command-4).
For more information about using the Piano Roll Editor, see Piano Roll Editor overview.