Surface tension  of a liquid is the force per unit length. In the illustration on the right, the rectangular frame, composed of three unmovable sides (black) that form a "U" shape, and a fourth movable side (blue) that can slide to the right. Surface tension will pull the blue bar to the left; the force  required to hold the movable side is proportional to the length  of the immobile side. Thus the ratio  depends only on the intrinsic properties of the liquid (composition, temperature, etc.), not on its geometry. For example, if the frame had a more complicated shape, the ratio , with  the length of the movable side and  the force required to stop it from sliding, is found to be the same for all shapes. We therefore define the surface tension as
The reason for the  is that the film has two sides (two surfaces), each of which contributes equally to the force; so the force contributed by a single side is .