The “Thin Blue Line” is an emblem used to show support of law enforcement officers. While the term has been around a long time, it’s believed to have been brought in the mainstream with the 1988 documentary of the same name.

Here’s a half double crochet beanie that implements a technique I call “chain tracks” where the blue line is a chain encased between 2 lines of yarn (aka “tracks”). Here is a video tutorial for this technique if needed: https://www.youtube.com/watch?v=QbAzi2KtX4M