The most obvious deformity with a flat foot occurs on the inner side of the foot and arch.  However this side of the foot does not exist in isolation and if the arch on the inner side flattens this may result in the following:

• heel moves outwards (valgus) 
• front part of the foot sits outwards at the midfoot 
• the Achilles tendon may also become tight as a result of the heel position 
• the forefoot may need to rotate inwards to balance the heel position

The correct medical term is a plano-valgus foot.  'Plano' refers to the flattened arch and ‘valgus’ to the heel position away from the midline of the body.