"Arches form a distinct class of two-dimensional structural elements that resist external loads through their profile (form). Compared to a beam element of the same span and subjected to the same load, the B.M. in an arch will be much smaller because of the negative B.M. due to the horizontal thrust at the supports (abutments). Graphical solution of arches is much simpler than the analytical solution, and is of adequate accuracy for practical purposes. The solution is based on Eddy's theorem on B.M. in arches and the concept of pressure (thrust) lines. ...In case the structure has the profile of the force polygon, the B.M. at any section will be zero... The structure, in such a case, will be subjected only to axial compression. Such a profile along the length of a beam or frame is known as the pressure line or line of thrust. ...the profile for a given system of forces, which would induce only compressive forces. The profile of a pressure line resembles an arch with linear segments; the profile is sometimes known as a linear arch."