§ VI. _b_ is a much rarer and less manageable type: for this evident reason, that while _a_ is the natural condition of a line rooted and strong beneath, but bent out by superincumbent weight, or nodding over in freedom, _b_ is yielding at the base and rigid at the summit. It has, however, some exquisite uses, especially in combination, as the reader may see by glancing in advance at the inner line of the profile 14 in Plate XV. § VII. _c_ is the leading convex or Doric type, as _a_ is the leading concave or Corinthian. Its relation to the best Greek Doric is exactly what the relation of _a_ is to the Corinthian; that is to say, the curvature must be taken from the straighter limb of the curve and added to the bolder bend, giving it a sudden turn inwards (as in the Corinthian a nod outwards), as the reader may see in the capital of the Parthenon in the British Museum, where the lower limb of the curve is _all but_ a right line.[84] But these Doric and Corinthian lines are mere varieties of the great families which are represented by the central lines _a_ and _c_, including not only the Doric capital, but all the small cornices formed by a slight increase of the curve of _c_, which are of so frequent occurrence in Greek ornaments. § VIII. _d_ is the Christian Doric, which I said (Chap. I., § XX.) was invented to replace the antique: it is the representative of the great Byzantine and Norman families of convex cornice and capital, and, next to the profile _a_, the most important of the four, being the best profile for the convex capital, as _a_ is for the concave; _a_ being the best expression of an elastic line inserted vertically in the shaft, and _d_ of an elastic line inserted horizontally and rising to meet vertical pressure. If the reader will glance at the arrangements of boughs of trees, he will find them commonly dividing into these two families, _a_ and _d_: they rise out of the trunk and nod from it as _a_, or they spring with sudden curvature out from it, and rise into sympathy with it, as at _d_; but they only accidentally display tendencies to the lines _b_ or _c_. Boughs which fall as they spring from the tree also describe the curve _d_ in the plurality of instances, but reversed in arrangement; their junction with the stem being at the top of it, their sprays bending out into rounder curvature.