Dominance, with its correlative, subordination, are everywhere present in architecture. In general, size and a central position, which usually go together, determine preeminence. The largest masses and those which occupy a central position inevitably rule the others. The towers and the facade dominate the exterior of a Gothic cathedral, the middle doorway is superior to those which flank it, and within, the central and larger nave dominates the smaller aisles on either side. When there are many dominant elements, as is necessarily the case in a large building, they must be unified by balance, if there are two, or by subordination to one of them, if three or more; otherwise, each claims to be the whole and the building falls apart into its members. There cannot well be three vertical dominant parts, because the central one makes a claim to preeminence which cannot be satisfied without superiority in size. A central member should, therefore, either be made larger than those flanking it, or else should be reduced to the status of a mere subordinate link between the others. In the horizontal division of a building into stories--as, for example, in the Palazzo Farnese near Rome--it is easier for the prominent parts to be equal, because they are better united by the evident contiguity of their masses, by their inclosure in a simple geometrical shape, and enframement between base and overhanging cornice. Yet here also we observe the tendency to make the middle larger or otherwise dominant, exemplified even in the building cited, where the central part is distinguished by the ornamental shield, upon which the attention is focused. When there are four horizontal divisions, our tendency is to divide them into groups of two; but unless this grouping is clearly marked by a molding or other such device, our purpose is defeated because each of the two can itself be divided into two parts, whence we get the four parts again, among which there is not sufficient unity. When, however, there are more than four stories, they cease to function as individuals and become members of a series, the rhythm of which creates the necessary unity. Even in this case, however, the tendency toward grouping into three with the middle dominant persists; for, as a rule, the stories are divided by moldings into three parts, of which the central part is the largest. Four equal stories are difficult because they at once resist an arrangement into threes and yet fall short of being the series which they suggest. When a series of stories is divided into three parts, a superior aesthetic effect is gained if the height of each story diminishes in some regular ratio from the bottom to the top, thus expressing the gradual overcoming of the downward force by the upward,--the rhythm becomes dynamical as well as kinematical.