A step-pattern is one which is formed of straight lines bent backwards and forwards at right angles so as to resemble a flight of steps. The lines are often arranged symmetrically round a centre, so as to make cruciform and swastika designs, and the different parts are also generally shaded alternately black and white on the principle of chequerwork. Step-patterns hardly ever occur in Christian Celtic art except on the enamelled bosses of metalwork and in a few of the illuminated MSS., such as the Lindisfarne Gospels, the St. Gall Gospels (Codex No. 51), the Gospels of MacRegol, the Book of Kells, and the Book of Durrow. The step-patterns in the MSS. so nearly resemble those on the enamelled bosses on the Ardagh Chalice, the Tara Brooch, and the Cross of Cong, that there can be but little doubt the illuminators copied their designs from the enamels. In the Pagan Celtic enamels the ornament is nearly always curvilinear; but in the Christian Celtic enamels it is rectilinear, the arrangement of the _cloisons_ being very similar to that on the Anglo-Saxon disc brooches incrusted with small slabs of garnet, glass, etc. Instances have already been given in a previous chapter of the use of step-patterns by the Pagan Celts on the engraved woodwork from the Glastonbury Marsh Village (p. 161). The only instances I have met with of step-patterns on the sculptured stones of the early Christian period in this country are at Bradford-on-Avon, Wilts; and Dysert O’Dea, Co. Clare. KEY-PATTERNS The term _key-pattern_ is used to describe a particular kind of rectilinear ornament which bears a certain amount of resemblance to the perforations in a key to allow it to pass the wards of a lock. The best-known key-pattern is the Greek fret. This is composed of what may be appropriately called straight-line spirals; that is to say, straight lines (or, to speak more accurately, narrow straight bars) bent round into a series of right angles in the same direction. The space between the lines (or narrow bars) is generally about the same width as that of the line itself. [Illustration: 1 2] [Illustration: 3 4 Key-patterns (1) Aberlady, Haddingtonshire (2) Abercorn, Linlithgowshire (3) St. Andrews, Fifeshire (4) Collieburn, Sutherlandshire ] The key-patterns used in Christian Celtic art may be classified as follows:— (1) _Square key-patterns_, in which the lines run horizontally and vertically parallel to the margins. (2) _Diagonal key-patterns_, in which the lines run vertically parallel to the right and left margins, and diagonally in two directions at an angle of 45° to the margins. (3) _Diaper key-patterns_, in which the lines run horizontally and vertically parallel to the margins, and diagonally in two directions at an angle of 45° to the margins.