As the curves are in each case accompanied by the actual figures for the value of each point on the curves, it is not necessary that one should be able to measure accurately for points falling between horizontal co-ordinate lines. Instead of consulting the vertical scale to get the value at any point, reference is made to the actual figures above the various points. These figures indicate a finer fluctuation in the curve than it is possible for the eye to appreciate even in curves plotted on very large sheets of paper. Figures, then, in conjunction with curves like those shown here, make it unnecessary to plot lines of such fine width or great accuracy as would be necessary if the figures were not shown above the curve.

In Fig. 207 the bottom card shows the zero line extended to the left over the figure seven at the lower left-hand corner of the ruled field. On the middle card the zero line has been extended to the right. This right-and-left extension of the zero line is not made until a card has been completely filled out. The extension of the zero line indicates the point at which cards are to be joined when laid in the horizontal position shown in Fig. 208. It will be noticed, in Fig. 208, that the cards are overlapped in such a way that the right-hand edge of the uppermost card coincides with the left-hand edge of the ruled field of the lower card, and that the zero line is continuous. The extension line drawn at the left of any lower card shows where the extension drawn across the right margin of the upper card is to be joined. Cards may be laid together in a horizontal position almost instantly, and correctly, when these joint lines are present as a guide.