The resistance a liquid exhibits to flow. Experimentally, the frictional force between two liquid layers moving past each other is proportional to area of the layers and the difference in flow speed between them. The constant of proportionality is called "viscosity" or "coefficient of viscosity", and is given the symbol . The time required for a liquid to drain out of a capillary tube is directly proportional to its viscosity. The poise is a non-SI unit frequently used to express viscosities.