The zero-velocity surfaces of the general planar three-body problem are constructed in form space, i.e., the factor space of the configuration space by transfer and rotation. Such a space is the space of congruent triangles, and the sphere in this space is similar triangles. The integral of energy in form space gives the equation of a zero-velocity surface. These surfaces can also be obtained based on the Sundman inequality. Such surfaces separate areas of possible motion from areas where motion is impossible.