A more complex type of harmony, since it admits of greater variety, is proportionality. Proportionality may be of various kinds. It may be merely the existence of a definite numerical relation between the dimensions of single parts, or the areas of various parts, of a building. This, in turn, may be either a simple arithmetical relation, such as exists between the parts of a Greek facade, each being some simple multiple of the unit or module; or a more complex relation like the Golden Section, where the smaller is to the larger dimension as the larger is to the sum of both; or like that which obtains when different parts form a geometrical series, where each is smaller or larger than the preceding by some fraction of the latter. The relation between the length and breadth of the facade of the Ducal Palace in Florence illustrates the Golden Section; the heights of the stories of the Peller House in Nuremberg form a geometrical series. This type of harmony is most complete when the proportion between the dimensions of the different parts is the same as that of the whole building,--by the ancients called _concinnitas_ because it produces a feeling akin to that of musical harmony. Dominance of a particular kind of line, horizontal or vertical, also gives harmony. Finally, harmony is secured by sameness of direction of line: the alignment of windows or parallelism between moldings dividing the surfaces of walls, for example. The relations, so seemingly mathematical, upon which architectural harmony is based, need not be exact, for two reasons: minor deviations are not perceptible, and even when perceptible, they give to the whole a feeling of life. Our experience with living things has taught us that, despite their orderliness, there is no exact mathematical regularity in their proportions; hence forms which cannot be precisely formulated are better fitted to symbolize life to us than the rigidly geometrical. The same experience has taught us that the curvilinear forms are closer to life than the angular; hence again the tendency, for aesthetic purposes, to introduce minute departures from the plumb-line and rule. There is, however, a type of life specifically human, the life of reason, which is best symbolized by mathematical relations; hence the Greeks, and all those who have followed the classical ideal, all who have had a passion for reason, have felt the circle and the square, and every other exact embodiment of clarity and intelligence, to be beautiful. In no other art has the passion for the intelligible been so perfectly expressed as in classical architecture.