Two more spaces were again allowed in 1912 in anticipation of even further upward progress in the curve. As joint lines may be drawn in continuation of any horizontal line on either the upper or the lower card, any amount of expansion in future years may be provided for. The curve cards are small enough to be very convenient to handle and yet they may be joined together in such a way as to provide for unlimited future growth.

CURVES FOR THE EXECUTIVE 273

The card for 1910 in Fig. 211 has a broad hne drawn by hand to show the zero hne. On the card for 1911 the scale begins at 400, and, because the scale does not begin at zero, the bottom of the co-ordinate field is marked with a wavy line. This wavy line is made very rapidly with a pen and serves the very useful purpose of safeguarding the reader from interpreting the curve as if the lower line on the field were zero. Whenever zero is not shown at the bottom of the ruled field, this wavy line should be used. Any card can thus be read independently, with safety so far as its interpretation from the zero point is concerned. When several cards are laid out together, and the zero line is shown on the left-hand card, as in Fig. 211, it is a simple matter for the eye to imagine the zero line extended to the right below the other cards, thus permitting easy interpretation of all cards.