雖然家裡沒有原文書，但看了這則 YouTube 影片之後才讓我發現原來印刷這麼的酷
以前書本大概只有注意到他的 ISBN 碼，也就是國際標準書號，在看了這影片之後
才發現原來原文書還有因為印刷的方便，擁有了一組特殊的神祕數字，而且還不一定排列方式都相同
會根據不同的出版社習慣，來印刷，這組數字稱為 Printer's key 它也稱為 number line。
而他主要的功能就是用來分辨這本書印刷的次數，數字只要看最小的數字就可以了，這是出版商在20世紀中期開始了這個約定。
(圖來至維基百科)
Why numbers are removed rather than added 為何是刪除數字而不是添加數字
With each successive reprint, the publisher needs to instruct the printer to change the impression number. In practice, if the plates (in offset printing) have been kept, a number can be erased, but nothing can be added. In this arrangement, all the printer must 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 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.[1] 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.
以上英文轉至維基百科，影片內也有中文解說
