(1) The prominence given to the margin or frame within which the whole design is enclosed. (2) The arrangement of the design within the margin in panels, each containing a complete piece of ornament. (3) The use of setting-out lines for the ornament, placed diagonally with regard to the margin. (4) The use of interlaced-work, step-patterns, key-patterns, spirals, and zoömorphs in combination. (5) The geometrical perfection of all the ornament. (6) The superiority of the decorative designs to the figure drawing. There are in the world two distinct schools of decorative art, one which entirely ignores the shape of the surface to be ornamented, and the other which allows the contour of the margin to influence the whole design. Japanese art belongs to the first of these, and Celtic art to the second. In the Irish illuminated MSS. the rectangular shape of the page determines the setting-out of the design, which is universally enclosed within a rectangular margin composed of lines of various thicknesses, or within an ornamental panelled frame. The only exception is in the case of the initial pages of the Four Gospels, where the margin is incomplete, so as to allow the extremities of the letters to project more nearly to the edge of the page. This prominence given to the margin often greatly influences the designs within it, more especially the key-patterns with diagonal setting-out lines. In sculptured stonework either roll-mouldings or flat bands form the margin, and in metalwork the margins are raised and the panels sunk. The panels within the margin are generally rectangular, but sometimes they are circular, annular, segmental, triangular, etc. The ornament in adjoining panels is seldom of a similar kind, and the patterns are often arranged on the principle of chequerwork, so that if there is a panel of interlaced-work at the left-hand upper corner of the page of a MS., and a panel of key-pattern at the left-hand lower corner, the order will be reversed on the opposite side of the page, and the key-pattern will be at the right-hand upper corner and the interlaced-work at the right-hand lower corner. The diagonal setting-out lines are chiefly confined to the key-patterns, and, as we shall see subsequently, are the origin of the peculiar form of Celtic key-pattern which was developed from the Greek fret.