But this may still not be enough, and the window may need three bars. Now the greatest stress is always on the centre of the window. If the three bars are equal in strength, as at _e_, the central bar is either too slight for its work, or the lateral bars too thick for theirs. Therefore, we must slightly increase the thickness of the central bar, and diminish that of the lateral ones, so as to obtain the arrangement at _f h_. If the window enlarge farther, each of the spaces _f g_, _g h_, is treated as the original space _a b_, and we have the groups of bars _k_ and _l_. So that, whatever the shape of the window, whatever the direction and number of the bars, there are to be central or main bars; second bars subordinated to them; third bars subordinated to the second, and so on to the number required. This is called the subordination of tracery, a system delightful to the eye and mind, owing to its anatomical framing and unity, and to its expression of the laws of good government in all fragile and unstable things. All tracery, therefore, which is not subordinated, is barbarous, in so far as this part of its structure is concerned. § VI. The next question will be the direction of the bars. The reader will understand at once, without any laborious proof, that a given area of glass, supported by its edges, is stronger in its resistance to violence when it is arranged in a long strip or band than in a square; and that, therefore, glass is generally to be arranged, especially in windows on a large scale, in oblong areas: and if the bars so dividing it be placed horizontally, they will have less power of supporting themselves, and will need to be thicker in consequence, than if placed vertically. As far, therefore, as the form of the window permits, they are to be vertical. § VII. But even when so placed, they cannot be trusted to support themselves beyond a certain height, but will need cross bars to steady them. Cross bars of stone are, therefore, to be introduced at necessary intervals, not to divide the glass, but to support the upright stone bars. The glass is always to be divided longitudinally as far as possible, and the upright bars which divide it supported at proper intervals. However high the window, it is almost impossible that it should require more than two cross bars. § VIII. It may sometimes happen that when tall windows are placed very close to each other for the sake of more light, the masonry between them may stand in need, or at least be the better of, some additional support. The cross bars of the windows may then be thickened, in order to bond the intermediate piers more strongly together, and if this thickness appear ungainly, it may be modified by decoration.