Why did half the books in this library mysteriously have the page number 7 underlined?

Twitter loves a good mystery, and this one sounds like it could have come from a game of Cluedo.

Library assistant Georgia recounted a tale from her job to Twitter on Tuesday when she sought to help an older lady who had found some strange pen markings in the books she was borrowing.

So there was a MYSTERY at the library today.

A wee old women came in and said "I've a question. Why does page 7 in all the books I take out have the 7 underlined in pen? It seems odd."
"What?" I say, thinking she might be a bit off her rocker. She showed me, and they did.

— Georgia Grainger BA, MLitt (@green_grainger) April 3, 2018

So who was doing this crafty misdemeanour? First of all, Georgia suspected the woman herself, but that was quickly put to rest.

I asked if she was doing it, she said she wasnt and showed me the new book she was getting out that she hadnt even had yet. It also had the 7 underlined! "I don't know, maybe someone really likes page 7?" I said, assuming of course that there is a serial killer in the library.

— Georgia Grainger BA, MLitt (@green_grainger) April 3, 2018

I checked some other books. Most didn't have it, but a lot in this genre did - they're "wee old women" books (romances set in wartime Britain etc). Lots of underlined 7s. The woman who pointed it out shrugged and went on her way, "just thought you should know".

— Georgia Grainger BA, MLitt (@green_grainger) April 3, 2018

The investigation was hotting up as Georgia searched for answers.

Luckily, the Sherlock to her Watson was on the way, in the form of her manager.

My manager came back from doing arts and crafts with some of the kids and I decide to tell her about the serial killer in the library.

And that’s how I found out that a lot of our elderly clientele have secret codes to mark which books they’ve read before.

— Georgia Grainger BA, MLitt (@green_grainger) April 3, 2018

Our computers do it automatically but many have been doing it since before that was possible, so Esther might underline page 7, while Anne might draw a little star on the last page, and Fred might put an “f” on the title page. Then when they pick it up, they can check!

— Georgia Grainger BA, MLitt (@green_grainger) April 3, 2018

Yep, you read that right. It’s a whole system for older people to check if they’ve read a book before. Mystery solved.

So, good news: there’s not a serial killer in the library whose MO include the number 7 and wartime romances. Bad news: people are defacing books rather than just asking us to scan them 😡

— Georgia Grainger BA, MLitt (@green_grainger) April 3, 2018

Georgia’s thread garnered a lot of attention, leading to fears of copy-cat pensioners in libraries across the country.

I'm now concerned that the amount of people enjoying this thread means there's going to be a new spate of readers using secret codes - apologies to librarians everywhere!

(although, in truth, I find it hard to be annoyed about it - better than torn pages and felt pen graffiti!)

— Georgia Grainger BA, MLitt (@green_grainger) April 4, 2018