A second principle of form is that shapes and sizes should be consistent. Its analysis has to do with the selective element in form and size as well as the problem of arranging these selected forms in the most harmonious and agreeable manner possible.

The bounding edges of forms or shapes are lines. These lines are made always at the junction of two colour tones or are formed by one colour touching another. Wherever this occurs a line is created. Every time colour tones change for any reason whatever, a new shape is begun or the shape considered begins to change and a lined condition exists.

Lines, as well as forms, are an important element in the consideration of composition. Good composition demands that these forms and lines should contain certain elements of likeness or harmony, and that they be so placed as to create this condition.

It is apparent then that too many colours, too much cut up in small areas, must result in the creation of too many shapes and lines. This tends to involve the problem in such a way that simplicity and repose in a room is well nigh impossible.

The kind of shapes and the direction of lines are as important as the number of them. Straight lines, which mark the shortest distance between two points, by their very nature seem simple, direct, forceful and somewhat structural. These qualities are the ones which the straight-line formation or construction should suggest, and where the feeling for them is not acute it is because lines of arrangement, as well as of pattern design, meet each other at obtuse and acute angles in such a way as to create a disagreeable feeling of opposition in line direction. Patterns in rugs and textiles often do this, as, in fact, the objects themselves are quite likely to do in the room arrangement in which the first principle of form - that is, consistent structural unity - is not conscientiously followed.

This effect of straight lines running in a slanting direction into other straight lines - excepting where the angles created are right angles - is ugly, non-structural and, consequently, usually uncomfortable in feeling Curved lines change their direction at every point.

There are in general three classes of these lines, as follows:

The arc of the circle changes its direction equally at every point. This is the most monotonous of curved lines, the simplest and most easily sensed. It lacks variety, and when used too frequently betrays lack of feeling for subtlety in line.

The arc of the ellipse, however, is more likely to change its direction at different points in the circumference, and presents a selective chance in line quite impossible in the arc of the circle. It is interesting, therefore, more subtle, and has greater aesthetic possibility.

The third class of curve is taken from the oval, and presents the greatest opportunity of all for fine relationships in variety of curve subtlety and in feeling for direction as well as for grace in line movement.

This curve of the oval appears in pottery and vase forms, in the general contour of ornament, and in other constructive curve-lined objects in the work of all nations where a fine aesthetic sense has been developed. The Greek, the Japanese, the High Renaissance in France, express their subtle relationships of curve in this type of line.

Mention of these three classes of curves is made here that one may become more sensitive to line as it appears in ornament and as it marks the boundary structural line of objects which are to be used as decorative motifs. The keener one's perception becomes in any field of expression the sooner will he realize the difference between the beautiful and the ugly, the aesthetic and the mechanical, the monotonous and the subtle. This perception is the key to the enjoyment of aesthetic relationships.

Forms, as they are created by lines, may also be characterized as straight-lined and curve-lined forms. The wall surface, the floor and the ceiling are generally of the first type. Some articles of furniture, pieces of pottery, pictures, clocks and other ornament are of the second class, and not infrequently a curved line in the form of an alcove, a bay window or arched ceiling forms a secondary consideration in a straight-lined figure.

When forms have a likeness which is more apparent than their difference, they at once become harmonious. A square or rectangle is bounded by four straight lines with four right angles, the only difference being that the square has four equal sides while the oblong has two pairs of equal sides, each pair differing from the other.

An oblong in a vertical position, like the side of a room, which is taller than its length, or a blank wall space between windows or adjacent to a door opening with a height exceeding its width, furnishes an opportunity for experiment with related and unrelated shapes.

A picture, for example, taller than it is wide, is a vertical oblong. Place it at equal distances from each of the sides of your wall space and about opposite the eye level, and you will sense a likeness in the ratio of the sides of the picture to the sides of the oblong space in which it is placed. This is related, harmonious and comfortable, if its size is good, in the space upon which it appears.

In the same position place a square picture and the effect is a little less pleasing, unless adjacent to or in some way related with it are other squares so that its distinctive form is not so apparent.

If one happens to have an elliptical picture and a round one, or even an elliptical vase, and a round clock, he should try each of these in the same position. He will see that the ellipse with a long vertical axis is more harmonious with the vertical space than if he should turn the ellipse so that the long axis would be horizontal. In that case one feels the opposition of the horizontal axis to the vertical line of the boundary space, and rebels against that structural motion, right and left, which is opposed to the vertical one of the wall space. This would be equally true, of course, of a horizontal oblong picture in the same space.