The structural properties of the graphite prismatic surfaces are studied using ab initio local-density-functional calculations. The armchair type prismatic surface is more stable than the zigzag prismatic, both for relaxed and unrelaxed configurations. After full ionic and electronic relaxation, the angle between carbon-carbon bonds increases at the zigzag surface. For the armchair surface, we found a strong contraction of the surface plane (8%). A simple bond scission model accounts for our results better than a tight-binding model.