The surface of any liquid is an interface between that liquid and some other medium. The top surface of a pond, for example, is an interface between the pond water and the air. Surface tension, then, is not a property of the liquid alone, but a property of the liquid's interface with another medium. If a liquid is in a container, then besides the liquid/air interface at its top surface, there is also an interface between the liquid and the walls of the container. The surface tension between the liquid and air is usually different (greater) than its surface tension with the walls of a container. And where the two surfaces meet, their geometry must be such that all forces balance.
Where the two surfaces meet, they form a contact angle, , which is the angle the tangent to the surface makes with the solid surface. Note that the angle is measured through the liquid, as shown in the diagrams above. The diagram to the right shows two examples. Tension forces are shown for the liquid–air interface, the liquid–solid interface, and the solid–air interface. The example on the left is where the difference between the liquid–solid and solid–air surface tension, , is less than the liquid–air surface tension, , but is nevertheless positive, that is
In the diagram, both the vertical and horizontal forces must cancel exactly at the contact point, known as equilibrium. The horizontal component of  is canceled by the adhesive force, .
The more telling balance of forces, though, is in the vertical direction. The vertical component of  must exactly cancel the difference of the forces along the solid surface, .
| rowspan="6" style="text-align:center;"|0°
| ethanol
| diethyl ether
| carbon tetrachloride
| glycerol
| acetic acid
| rowspan="2"|water
| paraffin wax
| style="text-align:center;"|107°
| silver
| style="text-align:center;"|90°
| rowspan="3"| methyl iodide
| soda-lime glass
| style="text-align:center;"|29°
| lead glass
| style="text-align:center;"|30°
| fused quartz
| style="text-align:center;"|33°
| mercury
| soda-lime glass
| style="text-align:center;"|140°
Since the forces are in direct proportion to their respective surface tensions, we also have:
where
*  is the liquid–solid surface tension,
*  is the liquid–air surface tension,
*  is the solid–air surface tension,
*  is the contact angle, where a concave meniscus has contact angle less than 90° and a convex meniscus has contact angle of greater than 90°.
This means that although the difference between the liquid–solid and solid–air surface tension,  , is difficult to measure directly, it can be inferred from the liquid–air surface tension, , and the equilibrium contact angle, , which is a function of the easily measurable advancing and receding contact angles (see main article contact angle).
This same relationship exists in the diagram on the right. But in this case we see that because the contact angle is less than 90°, the liquid–solid/solid–air surface tension difference must be negative: