But to our ancestors the important problem in first erecting the house was to secure the substantial support of the ridge-pole. Obviously the erection of two forked trees at either end of the ridge-pole made an excellent solution, but when the room was long this meant that the interior had to be cluttered up with interior posts. We find then that one of the primitive methods in England of eliminating the interior posts was the adoption of the cruck system of construction which is shown in Fig. 2. By selecting two bent trees and placing them together in a shape like a wish-bone, the ridge-pole could be well supported without interior columns. By placing cross-tie beams on these bent trees and extending them outward, the plates for supporting the lower ends of the rafters could be held in position. This permitted the carpenters to erect the exterior walls independently of the roof, a thing which they seem to have desired. There is another variation of the above method of supporting the ridge-pole, and that is shown in Fig. 3. Instead of selecting a bent tree, one was secured which was upright for a certain height, and then which bent to one side with a branch. By placing two of these trees together, a perfect end was formed for the house. However, this was not a very good type, since it meant the selecting of very unusual-shaped trees. [Illustration: ENGLISH POST & TRUSS CONSTRUCTION] For this reason the system of post-and-truss construction, which is shown in Fig. 4, was the natural outcome of the above. Diagonal bracing at the corners evidently was found to be useful in resisting high wind-storms, and it was usually employed. There apparently remained a distrust of masonry walls among the carpenters, for they continued to support the roofs entirely upon heavy timber framing, and records show that the exterior walls were built up after the roof-framing had been completed. There are evidences that the early types of walls, after the primitive woven brushwood walls proved insecure, were made like a barricade of trees; that is, they were merely a continuous line of vertically placed tree-trunks. This, of course, was a ruinously expensive type of wall when timber became scarce, and it is no wonder that it grew to a system of construction like that shown in Fig. 5. Even this required a good deal of wood, so that the filling of the space between the timbers rather logically became masonry or plaster on lath. However, the method of building shown in Fig. 5 has all of the elements of the system of construction used in framing modern exterior walls. The most important difference is in the size of the timbers used. [Illustration:TYPE OF ANCIENT WOODEN WALL ENGLISH HALF TIMBER CONSTRUCTION]