There are other cases where it is desirable to use this idea. For instance, wholesale dry goods firms when purchasing dry goods from eastern manufacturers request the latter when billing to use lot numbers furnished by the purchaser. It is also necessary for the manufacturers to bill the goods according to their own lot numbers. Hence it becomes necessary to have both the customer's lot number and their own on the sales sheet. This is accomplished by writing their own lot numbers directly upon the sales sheets at the left of the invoice and then making the invoice out in the regular way. This idea carries out the customer's wishes, and always gives a comparison of lot numbers to the manufacturers on their sales sheets. Flat-bed machines require wider invoice, Fig. 15. =Goods Purchased Outside.= It is possible to make a short cut in some lines of business where the goods sold are not kept in stock but are purchased from other firms in the same city. When placing the invoice in the billing machine over the sales sheet, requisition blanks in duplicate or triplicate can also be placed in the machine with the invoice, and the items which have to be ordered outside written first on the invoice. When all of these items have been entered on the invoice, the requisition blanks may be removed, and the invoice and sales sheet left in the billing machine, putting the remaining items which are to be shipped and charged on the invoice (sales sheet). This occurs where orders can be shipped complete. This plan can also be used when writing up the order, where the billing is done after the goods are shipped. It is to be remembered that there is a great distinction to be made in handling the billing work of firms who are able to fill their own orders complete, as contrasted with those firms who have to wait until goods are shipped in order to determine which items to bill. Some firms who always carry a complete stock are able to make up an invoice and order blank, a label for the express package, and a charge on sales sheet, simultaneously, because they know that they can ship every item called for, and consequently do not have to wait to see what items are shipped before billing them. [Illustration: Fig. 15. Form to Provide on Sales Sheet Information not on Invoice ] =Unit Billing.= The unit idea in billing has grown considerably in the last few years, notwithstanding that the size of the bills have to be uniform, and as large an invoice has to be used for one item as for a large bill. Many firms prefer the unit idea to the condensed sales sheet idea.