A Langmuir monolayer can be compressed or expanded by modifying its area with a moving barrier in a Langmuir film balance. If the surface tension of the interface is measured during the compression, a "compression isotherm" is obtained. This isotherm shows the variation of surface pressure (formula_2, where formula_3 is the surface tension of the interface before the monolayer is formed) with the area (the inverse of surface concentration formula_4). It is analogous with a 3D process in which pressure varies with volume.