The bike's body bends during use and it will scratch itself on the ring teeth and the chain. Sometimes older bikes have a bigger number of teeth on the middle chainring like this (s:m:l ring teeth ratio is about 32/48/52 teeth).
[numbered bike anatomy graph](https://i.ibb.co/7ynxRxQ/bicycle-anatomy-v1-0a.jpg)
chainring = 31;
bottom bracket spindle = 26;
bottom bracket shell = the hole where 27 is marked to go in
Yup, these dolphins are used for espionage, they’re not secret assassins. No dolphins in the UK tho.