 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 pick out the relevant blocks from the "sheet"; then the stack of blocks, which had been laboriously laid out when the page was first set up, could be inked for the reprint.In the case of a Linotype slug, the lowest number could be filed off and the slug reused.