Day and Zimmermann, of Philadelphia, are plotting a total of about eight thousand curves, most of the curves on a cumulative basis somewhat as shown in Fig. 134. In order to allow space enough for a cumulative curve (which naturally takes up more room than a curve plotted on a non-cumulative basis), the curves are plotted on cards 8}^ inches by 11 inches, and these cards are filed vertically in a tray desk so that a man sitting at the desk can instantly lay his hand on the proper card for any one of the eight thousand curves. Guide cards are of course used to make card location easier. To prevent cards being replaced at some wrong position in the file, Day and Zimmermann have worked out the clever scheme of notching in a similar manner the tops of all cards which are filed in any one division of the file. When cards are filed one after another the notches of individual cards form a groove in the group of cards. If any card should be filed out of place it would break the continuity of the groove and would instantly be noticed. In using the notched-card scheme two notches may, if desired, be placed in the top of each card, thus permitting many more group combinations than would be possible if only one notch were used. The notches may be cut with a special instrument of rather simple construction so as to insure their uniform spacing right or left from the upper corners of the card. A notch in the form of a segment of a circle is the most satisfactory.