The properties of fluid films depend on the nature of the film 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 film phase. For symmetrical films the first and last symbols are the same, e.g. A/W/A: water film in air, or W/O/W: oil film in water, whereas for unsymmetrical films these are different, e.g. W/O/A: oil film between water and air.