'Flat feet' (also called 'pes planus' or 'fallen arches') is a formal reference to a medical condition in which the arch (foot) of the foot collapses, with the entire sole (foot) of the foot coming into complete or near-complete contact with the ground. In some individuals (an estimated 20–30% of the general population) the arch simply never develops in one foot (unilaterally) or both feet (bilaterally).