The flat-bed billing machines are equipped with heavier type-bar springs than the cylinder machines. The operator in depressing the keys overcomes the additional resistance to the touch, due to the heavier type-bar springs, and strikes a heavy blow on the paper, making a good manifold copy in all cases. The operator on a cylinder billing machine strikes a heavier blow than usual to secure the heavy manifolding results. The heavier the blow, the clearer the result on a billing machine. The Underwood billing machine is shown in Fig. 3. [Illustration: Fig. 3. Underwood Billing Machine ] All typewriters equipped with special attachments for holding the sales sheets, invoices, and orders are called _cylinder billing machines_. The paper upon which the machine writes is held by, and passes over and around, a round rubber roll, the cylinder. This is in contrast to the flat-bed billing machines (or book typewriters) on which the paper lies flat on a rubber plate while the machine moves over the paper. =Development of Billing Machines.= This text has been prepared by taking the simplest forms of billing and order work, and leading up gradually to the more complicated forms. This is actually what took place in the improvement of office and factory work. A bill and a copy were first made—an extra sheet was made for some additional purpose. The advantages of doing two or three things at once led to further investigation. This resulted in still additional sheets being used for other purposes. Every time an additional sheet was added, the labor of typewriting that sheet separately was eliminated. Gradually it developed that it would be useful to copy a part only of the typewritten information on some of the under sheets. Means were found for accomplishing this. For instance, on an order form, it was desirable to have the prices show on the office copy, but not on the copy which was intended for the warehouse or factory. In some cases it was desirable to write all the information on the top copies and split up the information thereon on the sheets underneath. The final development of all the above ideas is embodied in the compound form, in which all sheets for the office, customer, warehouse, factory, shipping room, and loading platform are typewritten at one operation, and the invoice and duplicate finished as a separate operation. Wholesale and manufacturing lines are used mostly in this text to illustrate the evolution and improvement of billing and order work. Some of the conditions which have to be considered in installing office systems are as follows: Whether the orders are received from customers or salesmen or both, and which are in the majority. Whether the goods are carried in stock or manufactured or bought outside, or all of these. Whether the orders can be filled completely, or nearly so. Whether or not the factory may know the prices.