The essential difference between the key-patterns used by the Egyptians, Greeks, and Romans and those used by the Christian Celts consists in the introduction of diagonal lines by the latter. Square key-patterns (_i.e._ those of the Greek fret type) were very seldom used in Christian Celtic art. There is, however, a very good example on one of the crosses at Penmon, Anglesey. The first step in the evolution of the Celtic key-pattern was to turn the Greek fret round through an angle of 45° so as to make the lines run diagonally with regard to the margins instead of parallel to them. Key-patterns in this stage of development are to be seen in the Lindisfarne Gospels, the St. Gall Gospels (Codex No. 51), and an Anglian cross-shaft from Aberlady, Haddingtonshire, now at Carlowrie Castle, near Kirkliston, Midlothian. It will be observed, however, that the result of changing a square key-pattern into a diagonal one is to leave a series of unornamented triangles all round the edge (p. 280). When these triangles are filled in by bending the ends of the diagonal lines round through an angle of 45°, so as to run parallel to the margins, we get such a characteristically Celtic key-pattern as the one on the great cross-shaft at St. Andrews, Fifeshire. Lastly, when the opposite ends of the diagonal lines in the middle of the panel are bent round in a similar manner, the most typical of all the Celtic key-patterns is arrived at, of which there is a very good example on the erect cross-slab at Farr, Sutherlandshire. [Illustration: 1 2] [Illustration: 3 4 Key-patterns (1) Rosemarkie, Ross-shire (2) Farr, Sutherlandshire (3) Gattonside, Roxburghshire (4) Nigg, Ross-shire ] [Illustration: Shaft of Cross of Eiudon, at Golden Grove, Carmarthenshire] The filling in of the sharp corners made by the lines inclined to each other at an angle of 45°, with small black triangles (if in a MS.) or with sunk triangles (if on a sculptured stone) gives a decorative finish to the pattern, and still further adds to its distinctively Celtic character. Next to interlaced-work the key-pattern is the most common motive made use of in the decorative art of the Christian Celtic period. It occurs in nearly all the Hiberno-Saxon illuminated MSS. and on a large proportion of the sculptured monuments in Ireland, Scotland, and Wales. Key-patterns and interlaced-work in combination, but without any other decorative motive, may be seen on the Welsh crosses at Carew and Nevern, Pembrokeshire; Golden Grove, Carmarthenshire; and Llantwit Major and Margam, Glamorganshire. On the metalwork of the period key-patterns seldom occur, except on the bronze bells, on a strap-buckle from Islandbridge, near Dublin, and on the Crucifixion plaque of repoussé bronze from Athlone, now in the Dublin Museum. [Illustration: Detail of ornament on erect cross-slab at Rosemarkie, Ross-shire] SPIRAL ORNAMENT [Illustration: Methods of connecting spirals]