The rotary press printing method provided many advantages in the manufacture of U.S. postage stamps, primarily in the speed of production allowing the printing of stamps on rolls of paper rather than on the single sheet at a time of the flat plate method. It also allowed the gumming of stamps as a roll rather than by the individual sheet, again a big time-saver. However, when the rotary press rolls were cut down to sheets to be delivered to post offices, the sheets tended to curl back into their rolled form. This was not a problem with rotary coils which were sold in rolls, but it was a problem for postal clerks who needed the sheets of stamps to lie flat in their drawers. More pressing was the fact that it was exceptionally difficult to add pre-cancels to the curled sheets without tearing the sheets apart at the perforations, a problem that was alleviated somewhat as the Bureau took over the pre-canceling chore. Since un-gummed sheets from the rotary rolls did not curl, it was noted that the gum must be the cause of the curling. By simply adding "breaks" in the gum the curling was reduced considerably. The three1 main types of gum breakers and the numbering system as described by Armstrong are shown in the diagram below.