The free surface of a liquid kept in a container is curved. For example, when water is filled in a glass jar, it becomes concave but if you fill water in a paraffin wax container, the surface of water becomes convex. Similarly, when mercury is filled in a glass jar, its surface become convex. Thus, the shape of the liquid surface in a container depends on the nature of the liquid, material of container and the medium above free surface of the liquid.

Angle of contact is the angle that the tangential plane to the liquid surface makes with the tangential plane to the wall of the container, to the point of contact, as measured from within the liquid.

The angle of contact is acute for concave spherical meniscus, e.g. water with glass and obtuse (or greater than 90°) for convex spherical meniscus e.g. water in paraffin or mercury in glass tube.