The properties of fluid films depend on the nature of the @[email protected] phase and that of each of the two neighbouring bulk phases. These films should be described, where appropriate, by three capital letters such as A for air, W for water, O for oil, and S for solid, separated by solidi, the middle letter indicating the @[email protected] phase. For symmetrical films the first and last symbols are the same, e.g. A/W/A: water @[email protected] in air, or W/O/W: oil @[email protected] in water, whereas for unsymmetrical films these are different, e.g. W/O/A: oil @[email protected] between water and air.