Dalton's Law of Partial Pressures
Dalton's law of partial pressures states that the total pressure of a mixture of gases (the last tank in the picture) is equal to the sum of partial pressures exerted by each component gas (the first two tanks). This represents the limit of ideal behavior. Deviations can occur at higher pressures.
The units used for pressure are atmospheres (atm). The gases can be oxygen and nitrogen, the major components of air, for example.