The flat-bed machines were the first to use the combined machines. The plan in use was to place two recording registers on the billing machine and prove the total of each bill separately with one register, and accumulate the totals of all the bills for the day on the other register. Certain firms used extra recording registers for recording the totals of extra columns on the sales sheets in which they analyzed the sales according to classes of goods or some other classification. Some firms did not add the bills when making the extensions of the prices and quantities, but left the additions for the adding machine. The only draw-back to this plan was that if the operator copied a wrong total from the register dial it caused trouble. This fault, if fault it may be called, as it is not the fault of the machine, but of the operator, has been corrected by placing a subtracting device on the late models of combined billing and adding machines, whereby the subtracting device is thrown into action when the total of the bill is being written. If the correct amount of the bill is written on the invoice, the dial figures all turn to ciphers, which indicates that the amount has been correctly transcribed by the operator. Other improvements will likely follow in the many machines which are being placed upon the market. See Fig. 29 on Page 67. Some firms prefer to do all the billing as one operation, and use a separate or computing machine, as the case may be, for proving the additions at the end of the day. The reason is that by having two separate machines they can be used by different people during the day for different work. If computing machines are needed for adding, multiplying, and taking off discounts, the non-listing machines are recommended, as they are key-driven. If adding machines are needed when printed lists are wanted of all additions, then listing machines are recommended. The general public has recognized that each class of machine has its separate use. Some firms check the totals of listing machines with non-listing machines, in order to save time and paper. Non-listing machines are used for taking a record of rolls and yards of cloth during inventory time, and later are used to multiply the extensions. Some dry goods firms hire expert operators at a dollar an hour to do this class of work, as it can be done in one-fifth the time that it would otherwise take. For instance, an example in multiplication, such as _432_ × _235_, would take approximately ten seconds to write down, multiply, and set down the figures. With a multiplying machine mentioned above, it can be done in two seconds.