Some people say the words "pin" and "pen" differently, but other people say them the same way. When they do, it's called a "pin/pen" merger.
There are other vowel mergers in addition to the “pin/pen” and “cot/caught” mergers. There is also such a thing as a vowel split. There are even chains of vowel changes, where one vowel starts to be pronounced like another one, which then begins to be pronounced like yet another, in a domino-like sequence. We might talk about some of these other changes in a future episode, but if you can’t wait that long, you can find out more about them in the “Atlas of North American English.” I also recommend the most recent episode of the podcast Lingthusiasm—the podcast that’s “enthusiastic about linguistics,” which talks about chain shifts. It’s episode number 17, called “Vowel Gymnastics,” and I’ll put a link in the show notes. If you have funny stories about vowel variations leading to misunderstandings, I’d love to hear them! Leave a comment.