These arches consist of two circular arcs of different radius joining tangentially at a point. The lower arc is usually of considerably smaller radius to the upper arc. An example of this type of arch is shown in the following figure. The two smaller circles are placed horizontally of equal radius. Concentrating on one half of the arch, the point at which the arcs change is selected and a line projected through the point and circular arc centre are drawn back to a point where the distance from the joining point is equal to the required radius. The central arc can now be drawn. In order to form a correct angle, this centre must lie on the opposite side of the centreline to the respective arc. Defining one pair of arcs is all that is required as the others are defined by symmetry about the middle vertical line.
