Does a mercury barometer measure absolute pressure or gauge pressure?

I know that gauge pressure takes the atmospheric pressure as a zero point whereas absolute pressure measures the pressure in the open air or something along those lines. Does a barometer measure pressure within the tube itself? By that logic it is not measuring pressure in the open air but gauge pressure instead. If that is truly the case, then how are we able to obtain the atmospheric pressure using the gauge pressure?

• Have you tried a Wikipedia search on "mercury barometers"? Jan 29, 2021 at 0:38
• yes and i ran into some confusion due to semantics. on other websites i couldn't find a clear explanation. Jan 29, 2021 at 0:40