With each successive reprint , the publisher needs to instruct the printer to change the impression number, and the theory is that the printer is less likely to make a mistake if he is only removing the lowest number, rather than introducing a new number each time. With this arrangement, all the printer has to do is "rub off" the last number in sequence. Changing only the outer number requires the fewest possible changes to the page of characters, which means the smallest possible charge to the publisher. In the days of letterpress printing, where each character was a metal block, all the printer had to do was to physically pick out the relevant blocks from the "sheet" and then the stack of blocks, which would have been laboriously laid out when the page was first set up, could be inked up for the reprint. In the case of a Linotype slug, the lowest number could be filed off and the slug reused. For offset printing with metal plates, the number can be erased without damaging the rest of the plate. In each case, the change is minimal.