In the effort to acquire a sure taste for effects of unity, principality in form must be studied carefully and should be studied progressively, beginning with the simplest leaf and flower forms, wherein may be noted the way in which one part of a leaf is dominant, and one leaf in a spray of leaves. Simple examples of principality are found in the anthemion motive, in the volutes of Ionic capitals, in vases and pottery. More complex examples are afforded by many Persian rugs, in which the lanceolate ellipse of the medallion, reenforced by analogous lines in the corner pieces and the inner medallion, dominates the whole composition. In many of the Gothic cathedrals a single spire dominates the whole edifice and gives it unity, as does the dome of the Capitol at Washington.

By the method of repetition unity is insured through the recurrence of identical or more or less similar lines, shapes, hues, tones, textures, and proportions. The method can be applied to any room, under any conditions, and may be made to yield an effect either marked or slight, obvious or subtle, according to the manner in which it is employed. The unifying and esthetically pleasurable effect of repetition has a double basis. It is in part physiological, and is due to the fact that the perception of like or repeated elements involves little muscular effort, whereas the perception of unlike elements necessitates a constant movement and adjustment of the eye. Psychologically, repetition is associated in the mind with the ideas of succession, order and regularity, and hence with the sense of repose and quiet well-being which always results from order and regularity in the affairs of life. On the other hand, change and non-succession are associated with the ideas of disorder, irregularity and disquietude. Thus the recurrence of similar lines and shapes, as in the repetition of a pleasing ornamental motive or the mechanical repetition of an inconspicuous pattern on the walls or floor, affects the mind, as does the recurrence of musical cadences or the rhythmic repetition of rhymed syllables, with a sense of quietude, order, and calm unity.

In good decoration the method of repetition is employed in three forms: (a) in its simplest and most common form, as repeating diaper pattern, which is used in wall papers, in damasks, tapestries and other drapery and upholstery stuffs, in all-over carpets and in many ornamental plaster ceilings, to cover entire surfaces with the same motive repeated continuously; (b) in its most obvious form, as symmetrical repetition, wherein each color, outline or mass on one side of a real or ideal center is balanced by a like color, outline or mass on the other side; and (c) in its most subtle form, as the recurrence, in many and often in widely separated parts of a composition, of identical or similar lines, shapes, colors or significances.

The use and decorative value of diaper pattern will be discussed in the chapters on proportion and excellence in design, while symmetry will be studied in the chapter on balance. This latter form of repetition has a marked unifying power. Because the like elements lie immediately before the eye, symmetry makes it easier for us to see and grasp the significance of things than is possible in non-symmetrical arrangements of decorative features. Thus a pair of identical candlesticks, placed at equal distances from the center of a mantel, would have an effect upon the mind at once unifying and obvious. Symmetrical repetition, whether it appears in the two halves of the same unit, as in a chair, a rug, or a window hung with draperies, or in arrangements of several units as groups, as in the case of a console table with a mirror above it and identical chairs at equal distances from either end, is never subtle. Its effect is always obvious and always formal, and when over-emphasized, as may very easily happen, it results in over-formality and stiffness.

The use of recurring lines and shapes and echoed colors lies at the basis of all fine work in interior decoration, as in architecture and the other visual arts. The constant repetition of similar combinations in both outline and ornament constitutes a large part of the charm of what we call the period styles. In the nature of things it must constitute a large part of the charm of any beautiful room, since, as we have seen, it provides one of the conditions in the absence of which beauty cannot be made to appear. Thus the repetition of similar straight lines, as in the architecture and decoration of Craftsman houses, makes for unity; and so, far more subtly, does the repetition of identical or similar curves.

For example, the cabriole legs found in Louis XV, Dutch, and Queen Anne furniture, and in many fine Chippendale pieces, and illustrated in Figure 10, are based on the cyma recta, or line of beauty curve, and in a room in which important pieces of furniture of this type are used a subtle effect of unity in variety can be produced by repeating variations of this same curve in the outline or ornamental details of lambrequins, mirror tops, lighting fixtures, lamps, candlesticks, vases, mantel clocks, andirons and firescreens; in the legs and finials of bookcase, desk or cabinet; in the border stripes of rugs, the seats and backs of chairs, the molding of cornice, trim and picture frames. Of course this does not mean that this curve must appear in all these situations. In fact, as will be developed in the chapter on Contrast, over-emphasis of any type of line, however pleasing in itself, results in monotony and the loss of decorative charm. What it does mean is that the curve must be repeated a good many times and in various situations in order to yield a marked yet subtle effect of unity, and that, within reasonable limits, every such repetition will add to the mind's pleasure.

In the same way the elliptical medallion of a rug may be repeated in an elliptical table, in a mirror, in chair-backs, vases, lamps, candlesticks, small easel pictures, ferneries or tea tables; and suggested more or less definitely by such features as the arc of a half-elliptical wall table, the tops of book-blocks, or the defining curves of valances or tied-back draperies. The oval of many Hepplewhite chair-backs may be echoed in bowls, lamps, Italian candlesticks, andirons, bellows, in a lamp surmounted by a mushroom shade, in the echinus or egg and dart molding. A round dining table may be related to an oblong dining room by means of an oblong rug with a rounded elliptical medallion, by hangings or a frieze having a pattern based on the circle or hexagon, by a round bowl of flowers, a round Sheffield tray, a Lazy Susan, or by the wheels of a tea wagon. Similar triangles may appear in pediments, lamp shades or mantel clocks, as well as in groupings of furniture and small decorative objects; and similar oblongs in ceilings, wall spaces, windows, doors, rugs, table tops, pictures or books.