Centres for the left-hand side are simply obtained by projecting horizontally to points equidistant from the centre line. In the construction of large arch centres, the loading becomes correspondingly heavier; consequently more consideration must be given to ensure that the erection is sound in design principle and construction. The use of arches to span openings in this country dates from Roman times. Although steel and reinforced concrete provided a cheaper and generally more economic alternative, arches are essential to buildings of certain styles and construction, especially those with brick or stone facings. There is a variety of curves on which arches may be based, but those used are mainly the circle, the ellipse and the parabola. The centre for thin walls is merely a flat timber beam of suitable width to give support and with the top surface slightly cambered as required.