Plate X., Figs. 1 and 2, show two ordinary _principals_ or _trusses_. These are in no sense Gothic, but are here given to more clearly explain the principles of roof construction. The common rafters (c.r.) bearing the covering are of light timber, tending to bend under the weight; to prevent this _purlins_ (P.), stout beams, are placed at suitable intervals, and these are carried at their ends by the _roof trusses_. The _tie-beam_ (t.b) is the chief beam of the truss. The _principal rafters_ (p.r.) are framed into it and into the heads of the _King_ and _Queen posts_. In the _Queen-post-truss_ the _collar_ unites the Queen posts; _struts_ and _cross-braces_ complete the structure. In a properly constructed roof-truss all the stresses are neutralised in the truss itself, and the whole framework rests as a dead weight upon the walls without any lateral thrust to force them out of the vertical. The King-post-truss is suitable to roofs up to 30 feet span, the Queen-post-truss to 40 feet. _Open timber roofs_ have their construction visible from the interior. Plate X., Fig. 8, shows a crude _Queen-post-truss_ in an old Worcestershire church, in which the tie-beam has been chosen from a bent log so as to prevent its bending under the roof load. In the trusses (Plate X., Figs. 1, 2) the King and Queen posts act like the keystones of an arch, so that by bolting or strapping up with ironwork at points s.s. these _posts_ are put into a state of tension, the tie-beam (t.b.) is pulled up to a _camber_, or curve, and is also in tension. In the crude Queen-post-truss (Plate X., Fig. 8) these conditions are reversed, for the tie-beam _supports_ the Queen posts. In Plate X., Figs. 1 and 2, all the spaces in these trusses are triangular, a fact which ensures stability where the parts are of proper strength and properly united. Plate X., Fig. 4, is a trussed rafter roof requiring no purlins nor principal--each common rafter is a truss. This kind of roof is suitable only to small spans; its weakest part is from a. to b. In the roof-truss, or principal, the Gothic architects objected to the tie-beam as an obstruction to the sense of loftiness, so desirable in the church interior, and therefore invented the _hammer-beam_ principal (Plate X., Fig. 7), which resembles a Queen-post-truss, having a collar and King post. The hammer-beams (h.b.) are substitutes for the tie-beam. In this principal the space between the collar and the apex of the roof is satisfactorily trussed, but from the collar downwards the _Queen posts_ (Q.p.) and _wall posts_ (w.p.) with their _braces_ (b.) become mere brackets supporting the small King-post-truss above, bringing its load as low as possible on to the walls, so that their weight of masonry and buttresses may effectually resist the lateral thrust of the roof upon them.