This would allow the sales to be trebled in volume before the curve would extend over the top of the ruled portion and necessitate a change in scale or a step upward so that the zero line could no longer be shown at the bottom of the card. It is well to start all curves for output or sales at about one-third of the height of the card so as to allow plenty of room for future expansion. Curves for expenses per unit, however.

CURVES FOR THE EXECUTIVE 275

may be started well up on the card if there is any hope whatever of reducing the unit expenses in future years.

The man who plots curves on the cards described here keeps a supply of the printed cards in each of the two rulings of seven spaces high and ten spaces high. When starting any new curves he uses whichever of the two cards gives the scale best suited to his purpose. The cost of carrying two different kinds of ruled cards on hand is negligible compared with the great convenience resulting.

The ruling of the cards in which the vertical spaces are either T-inch or ■g--inch high permits the use of an engineer's scale in fortieths or sixtieths of an inch, if it should ever be desired to locate plotted points on the cards with very great accuracy. The engineers' scale in fortieths or sixtieths of an inch gives ten divisions to each space between the horizontal lines on the card and makes it possible to locate each plotted point with a very finely sharpened lead pencil or a needle. Practice, however, proves that there is no necessity for using an engineer's scale in plotting curve points on the cards here described. The man doing the plotting learns very quickly to locate the points by using only the eye and a hard lead-pencil, so that the points are practically as accurate as if spaced with an engineer's scale and a needle point.