Abacus (Lat. from ἄβαξ, a board). This name is applied in architecture to the uppermost member or division of a capital.

1. In the Grecian and Roman orders it is a very essential feature.

In the Grecian Doric the abacus has simply the form of a square tile without either chamfer or moulding.

In the Roman Doric it has the addition of an ogee and fillet round the upper edge.

In the Tuscan a plain fillet with a simple cavetto under it is used instead of the ogee and fillet.

In all these orders the abacus is of considerable thickness, and the moulding round the upper edge is called the cimatium of the abacus.

In the Grecian Ionic it is worked very much thinner, consisting of an ovolo or ogee, generally without any fillet above it, and is sometimes sculptured.

In the Roman Ionic it consists of an ogee or ovolo with a fillet above it.

In all the preceding orders the abacus is worked square, but in the modern Ionic, the Corinthian, and the Composite, the sides are hollowed, and the angles, with some few exceptions in the Corinthian order, truncated. The mouldings used on the modern Ionic vary, but an ogee and fillet like the Roman are the most common. In the Corinthian and Composite orders the mouldings consist of an ovolo on the upper edge, with a fillet and cavetto beneath.

2. In the architecture of the Middle Ages, the abacus still remains an important feature, although its form and proportions are not regulated by the same arbitrary laws as in the classical orders: in the earlier styles there is almost invariabily clear line of separation to mark the abacus as a distinct division of the capital; but as Gothic architecture advanced with its accompanying variety of mouldings, the abacus was subject to the same capricious changes as all the other features of the successive styles, and there is often no really distinguishable line of separation between it and the rest of the capital.

It not unfrequently happens that the abacus is nearly or quite the only part of a capital on which mouldings can be found to show its date; it is therefore deserving of close attention.

In early buildings of the style spoken of as being, perhaps, Saxon, that is, belonging to the 11th century, the abacus is, in general, merely a long, flat stone without chamfer or moulding; but it sometimes varies, and occasionally bears some resemblance to the Norman form.

The Norman abacus is flat on the top and generally square in the earlier part of the style, with a plain chamber on the lower edge, or a hollow is used instead. As the style advanced, other mouldings were introduced, and in rich buildings occasionally several are found combined; it is very usual to find the hollow on the lower edge of the abacus surmounted by a small channel or a bead. If the top of the abacus is not flat, it is a sign that it is verging to the succeeding style.

In the Early English style the abacus is most commonly circular; it is, however, sometimes octagonal, and occasionally square, but not frequently in England, except early in this style. The most characteristic mouldings are deep hollows and overhanging rounds; in general, the mouldings in this style have considerable projections with deep and distinct hollows between them.

In the Decorated style, the form of the abacus is either circular or polygonal, very frequently octagonal. The circular abacus is especially an English feature; the octagonal abacus being most common on the Continent, especially in France. Hollows are not so frequently to be found, nor are they in general, when used, so deeply cut; the mouldings and the modes of combing them vary considerably, but rounds are common, particularly a roll or scroll-moulding, the upper half of which projects and overlaps the lower, as in Merton College Chapel; this moulding may be considered as characteristic of the Decorated style; although it is to be met with in late Early English work. The round mouldings often have fillets worked on them, and these again are also found in Early English work.

In the Perpendicular style the abacus is sometimes circular, but generally octagonal, even when the shaft and lower part of the capital are circular; when octagonal, particularly in work of late date, the sides are often slightly hollowed in this style the mouldings are not generally much undercut, nor are they so much varied as in the Decorated. A very usual form for the abacus consists of a waved moulding (of rounds and hollows united without forming angles) with a bead under it, as at Croydon, Surrey. The most prominent part of this moulding is sometimes worked flat, as a fillet, which then divides it into two ogees, the upper being reversed the ogee may be considered as characteristic of the Perpendicular capital. The top of the abacus is sometimes splayed and occasionally hollowed out.

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