The weight of all of an arch's parts holds it together, making building difficult. One solution is to construct a frame (historically, of wood) that precisely matches the shape of the arch's bottom. This is referred to as the "centre" or "centring." The frame is then filled with concrete, leaving only four openings for light and air. An arch has two main forms: the half-dome and the pointed arch. In both cases, the weight of the structure above forces the members down into the ground or supports. If the members were not restrained from rising up out of the ground, they would collapse the arch itself. Half-domes and elliptical arches tend to be more stable than pointed arches because there is more surface area connecting the top and bottom of the structure. This means less stress on any one part of either form of arch. Pointed arches have a very slender base and tall point, where they connect to the wall surfaces below. Because there is no horizontal support beneath the points, they tend to be less stable than other shapes. However, if the points were to touch the ground or some other horizontal support, they could remain upright without collapsing. Domed arches have a floor or base under them where the depth of the floor or base equals the height of the arch.