3.8 Using the Metronome
You can set up and use a metronome when using REAPER for recording.
Kind: concept (user-guide-section) Chapter: 3 Recording Audio and MIDI Source: REAPER User Guide v7.70

You can set up and use a metronome when using REAPER for recording. Use the Options, Metronome enabled command to turn it on and off (or left-click the metronome button on the toolbar), and the
Options, Metronome/ pre-roll settings
command to set up your metronome (or right-click on the metronome button on the toolbar).