From what has been said it will be seen that it is impossible to establish a standard sectional profile to suit all conditions. The best one for the majority of conditions, and the one most commonly employed, is a polycentric figure in which the number of centers and the length of the radii are fixed by the engineer to meet the particular conditions which exist. In a general way this form of center may be considered as composed of two parts symmetrical in respect to the vertical axis. Fig. 7 shows such a profile, in which _DH_ is the vertical axis. The section is unsymmetrical in respect to the horizontal axis _GE_. The upper part forming the roof arch is usually a semi-circle or semi-oval, while the lower part, comprising the side walls and invert of floor, varies greatly in outline. Sometimes the side walls are vertical and the invert is omitted, as shown by Fig. 8; and sometimes the side walls are inclined, with their bottoms braced apart by the invert, as shown by Fig. 9. In more treacherous soils the side walls are curved, and are connected by small curved sections to the invert, as shown by Fig. 10. In the last example the side walls are commonly called skewbacks, and the lower part of the section is a polycentric figure like the upper part, but dissimilar in form. In a tunnel section whose profile is composed entirely of arcs the following conditions are essential: The centers of the springer arcs _Ga_ and _Ea′_, Fig. 7, must be located on the line _GE_; the center of the roof arc _bDb′_ must be located on the axis _HD_; the total number of centers must be an odd number; the radii of the succeeding arcs from _G_ toward _D_ and _E_ toward _D_ must decrease in length, and finally the sum of the angles subtended by the several arcs must equal 180°. [Illustration: ~Fig. 8~ ~Fig. 9~ ~Fig. 10~ FIGS. 8 to 10.--Typical Sectional Profiles for Tunnel.] =Dimensions of Section.=--The dimensions to be given to the cross-section of a tunnel depend upon the purpose for which it is to be used. Whatever the purpose of the tunnel, the following three points have to be considered in determining the size of its cross-section: (1) The size of clear opening required; (2) the thickness of lining masonry necessary; and (3) the decrease in the clear opening from the deformation of the lining.