If the two vases which are represented in the view by vertical and horizontal, straight and curved lines, were actually before us you would have difficulty in finding any vertical lines, and the horizontal lines would turn out to be circles. The lines in the view mark the apparent terminations of the surfaces. For purposes of study, however, you must regard objects of three dimensions as bounded by lines, just as they appear in photographs, drawings, or other flat representations, geometric or perspective. In regarding objects from the point of view of decoration there is still another element to be considered; that is, the element of material, the substance of which objects consist, for it is evident that the ornament which would be appropriate to wood, for instance, 
Fig. 4. might not be appropriate to metal or to stone. The element of material is of great importance in practical decoration, but of less importance in theoretical decoration. Lines and surfaces are therefore the two chief elements of decoration to be considered at present. Color, being an element of an entirely independent nature, will not be considered at all.
First, lines. The lines down one side of an object may be called the profile of the object, while the lines surrounding the object may be called the contour or outline of the object.
Profiles and outlines are made up of any number of straight and curved lines connected at any and every variety of angle. The view (Fig. 2) shows a few possibilities of combination of lines into profiles. The particular thing to be observed in these profiles is that individual curves are preceded or followed by curves which curve in the same direction or in the opposite direction—that is, regarding the curves as concave or convex from a given side of the profile, sometimes a concave curve meets a concave curve, sometimes it meets a convex curve. In these particular profiles the straight lines which unite the curves are so small and so insignificant that they appear as mere connections. Where the adjoining curves are homogeneous the connection is called con-
