9.6 Show/Hide Regions/Markers in Grid
You can choose whether or not to show project region/marker lines and/or time signature changes on the grid by enabling or disabling the relevant preferences.
Kind: concept (user-guide-section) Chapter: 9 Tempo Based Production, Loops, Markers and Regions Source: REAPER User Guide v7.70
You can choose whether or not to show project region/marker lines and/or time signature changes on the grid by enabling or disabling the relevant preferences. These can be found on the Options, Preferences, Appearance, Ruler Grid settings page.

If your project does not include any time signature or tempo changes, you can gain a little more screen real estate by choosing Ruler display, Hide tempo/time signature area if project has no time signature/ tempo changes from the arrange view context menu.