Why are these 3 letters on almost all of my zippers?

How do zippers work? Sponsored by Hostinger. Host OpenClaw on a Hostinger VPS plan: https://www.hostinger.com/veritasiumopenclaw - and use code 'veritasium' for an extra discount. If you’re looking for a molecular modelling kit, try Snatoms, a kit I invented where the atoms snap together magnetically - https://ve42.co/SnatomsV Sign up for the Veritasium newsletter for weekly science updates - https://ve42.co/Newsletter ▀▀▀ A huge thank you to Prof. Robert Friedel for his invaluable expertise and contributions to this video. Check out Friedel’s book here - http://ve42.co/friedel-zipper Massive thanks to IPFL for fabricating our giant zipper - https://ve42.co/ipfl-fabrication A huge thank you to Emily Zhang for her advice on how to best structure this story. Check out her new channel here: @rabbithole_video The giant zipper was adapted from an original design by LoboCNC - https://ve42.co/fidget-zipper We’d also like to thank everyone at YKK who helped organise our call, especially Sophia Seco, Engin Ertugrul, Christelle Malot and Matthew Rawstron. Finally, a big thank you to Christopher Anderson, Anna Oros and Pressley Stevens of the Allegheny College Library for sending us footage of Whitcomb Judson’s original fastener in action. ▀▀▀ 0:00 The First Zipper 2:51 Who invented the zipper? 4:20 How does a zipper work? 5:51 The Zipper Maker Machine 7:46 Why is it called a zipper? 9:32 The Novelty Of A Zipper 12:57 The Most Popular Zipper Doesn’t Have Teeth 14:08 Zippers Have A Lock 15:31 Why do zippers have YKK on them? 18:39 How To Fix A Zipper ▀▀▀ References and Visuals can be found here - http://ve42.co/ZippersRefs ▀▀▀ Special thanks to our Patreon supporters: Adam Foreman, Albert Wenger, Alex Porter, Alexander Tamas, André Powell, Anton Ragin, armedtoe, Balkrishna Heroor, Bertrand Serlet, Blake Byers, Bruce, Charles Ian Norman Venn, Daniel Martins, Data Don, Dave Kircher, David Johnston, David Tseng, EJ Alexandra, Evgeny Skvortsov, Garrett Mueller, Gnare, Hayden Christensen, Hong Thai Le, Ibby Hadeed, Jeromy Johnson, Jesse Brandsoy, Jon Jamison, Juan Benet, Kelcey Steele, KeyWestr, Kyi, Lee Redden, Marinus Kuivenhoven, Mark Heising, Martin Paull, Meekay, meg noah, Michael Krugman, Moebiusol - Cristian, Orlando Bassotto, Parsee Health, Paul Peijzel, Richard Sundvall, Robson, Sam Lutfi, Shalva Bukia, Sinan Taifour, Tj Steyn, Ubiquity Ventures, Vahe Andonians, wolfee ▀▀▀ Writers: Daniel Leonard & Gregor Čavlović Producer & Director: Daniel Leonard Presenter: Gregor Čavlović Editor: Trenton Oliver Animators: Domonkos Józsa, Fabio Albertelli & Andrew Neet Illustrators: Jakub Misiek & Emma Wright Additional Editor: James Stuart Researchers: Callum Cuttle & Sophia Rose Camera Operators: Gregor Čavlović & Henry van Dyck Thumbnail Designers: Abdallah Rabah, Ren Hurley & Ben Powell Production Team: Josh Pitt, Matthew Cavanagh, Anna Milkovic, Katy Southwood, Glen Griffiths & Jess Bishop-Laggett Executive Producers: Derek Muller, Gregor Čavlović & Casper Mebius Additional video/photos supplied by Getty Images & Storyblocks Music from Epidemic Sound