In order to obtain the true shape of the segment the elevation should be divided by horizontal lines drawn at the points of marked change in curvature, and these can be lettered or numbered for identification. The distance between each of these lines should be measured and set off on the new centre line, and then these can be used for drawing the parallels through. The various diameters can be determined from the corresponding lines on the elevation. Lines drawn through the points thus obtained will give the required segment or area on the flat. [Illustration: No. 227. Method of obtaining a Segment of one-fifth of Vase.] If other divisions than those deducible from three or six are required, it will be necessary to draw also the plan curves from which the division can be obtained. Assuming that the elevation has been vertically divided as before, and the plan to be circular, a circle should be struck which is to represent the largest diameter and its circumference divided into the required number of parts. Lines are then drawn through to the centre. On the same centre other circles are struck with radii equal to the remaining horizontals, and each identified with the corresponding number or letter. The heights are obtained as before, and the diameters of the variations in the curvature can be ascertained by measuring round each of the plan curves in succession. In the case of the plan being other than circular, the same rules apply, but the different plans would have to be drawn in each individual example. Owing to the effect of perspective on rounded shapes, it is undesirable to employ the human figure, unless in bold relief, and then only on straight or slightly curved profiles. CHAPTER VI DEVELOPMENT OF CONVENTIONAL ORNAMENT The term Convention is applied to decoration in which there is distinct evidence of artistic restraint, which may be purely æsthetic or due to technical conditions. _Outline Drawing_ The rendering of any form in outline is probably the simplest form of convention, which is generally accepted through tradition as representation. Though the objects so depicted really depend on light, shade, and local colour for their appearance. Such outline drawings may be in other respects realistic, but a further degree of convention is the desirable elimination of perspective where it is unsuitable to the effect desired. For instance, in silhouette, profile renderings only are intelligible, and in delicate bas-relief modelling any foreshortening should be avoided if confusing to the effect.