https://mashable.com/2018/04/05/library-mystery-codes-twitter-thread/#7Hr7Gi... Elderly library patrons underline page 7 in war time Britain romantic story books. Some put a dot or a star etc to know if they have read it earlier. This reminds me of the practice of writing accession number or putting library rubber stamp / seal on designated pages of (text) (reference) books likely to be stolen. I used to put library stamp on p 107, 207 etc. If a book is stolen and recovered, even if title page having library seal / number is torn, p 107 / p 207 would remain in tact. Dr P Vyasamoorthy 040-27846631 / 09490804278