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Insert threaded comment

Windows shortcut 
CtrlShiftF2
Mac shortcut 
FnF2

If there is no comment in the active cell, a new threaded comment will be created. If a comment already exists in the cell, the comment box will open.

Note: In Excel 365, the original "comment" was renamed to "note" and a new "threaded comment" was added to allow back and forth comments by reviewers.

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