It's related to how reading and the human eye work. For each eye fixation, you take in about 5-10 characters. The optimal line length for books is about 60-75 c

It's related to how reading and the human eye work. For each eye fixation, you take in about 5-10 characters. The optimal line length for books is about 60-75 characters, which takes somewhere between 6-15 fixation to scan. Code is not like books: most lines will be shorter than the "limit", there's indentation, you're typically reading monospaced characters on a screen, and you have syntax highlighting, etc. So, bumping the length up to 80 characters is still okay, as is the occasional outlier. But regularly writing 100-120 characters-long LoCs will definitely impact readability.