This is due chiefly to the elasticity of the scheme. The duplicate, triplicate, or quadruplicate of the invoice can be sorted in any desirable way. It is especially convenient for bookkeepers to sort duplicates of the invoices alphabetically, and save a great deal of time in posting to loose-leaf ledgers arranged alphabetically, in the same manner. (In mentioning loose-leaf ledgers, card-ledgers are always included, as the same principles are applied to one as to the other, as far as accounting methods are concerned.) It is easier to handle unit billing forms on the typewriters with billing attachments, which is an added reason that many firms prefer to use them. Some of these forms are shown in Fig. 16. Another idea to be recommended is the color scheme, whereby each copy of the invoice is manifolded onto a different-colored piece of paper. In sorting the various copies for different departments, different colors will greatly facilitate the recognition of various sheets, and the uses or departments for which each is intended. ORDER WORK OF WHOLESALE GROCERS There are hardly two firms who handle their order work alike. The first consideration in treating order methods is to distinguish the classes of business in which the orders are made up ready for execution by the salesman, from those which are received from customers and transcribed on typewriters. The first class will be discussed separately from the second. Wholesale grocers, druggists, and similar lines receive the great majority of their orders from the salesmen in the field. After the orders are opened, the first step is to stamp on each order a number with an automatic numbering machine. After this is done they are copied into an order register as follows: [Illustration: Fig. 16. Samples of Unit Billing. _Remington Typewriter Co._ ] 1012 John Smith & Co., Plainfield, N. J. 1013 A. B. Jones & Bro., Providence, R. I. 1014 U. J. Benedict, Elmira, N. Y. 1015 Grace Barnes & Co., Alliance, Ohio. After the orders are filled and charged, they are checked on the order register. About once a week all of the unchecked numbers on the order register are compared with the unfilled orders. If an order should become lost, the fact would not remain unknown longer than one week's time. It should always be remembered that in case an order is not completely filled and is "back ordered," the back order should always show the original order number in order to prevent confusion. A little practical experience will quickly show how wrong it is to use a new number for a back order.