What is the reason / origin of textwidth = 78 (and not 80)?

Question

In vim, it seems common practice to set the text width to 78 rather than (as I expected) 80 characters. I see this everywhere (help pages, modeling, vimrcs, etc.).

Just curious, does anyone know why this is?

Edit: I understand the source / argument for setting the text width to 80 characters. It's just curious why in normal practice, set the width of 2 characters to less than 80.

Answer 1

As noted in the comments, RFC2822 (Internet message format or "email") recommends a line length of 78 (excluding CRLF). The rationale he gives is

A more conservative recommendation of 78 characters is to allow for many user interface implementations that display these messages, which may crop or catastrophically wrap displaying more than 78 characters per line

I don’t think RFC2822 is why you see the length of 78 character lines so often, but I suspect the excuse is similar. In the era of reign, terminals were typically 80-column devices. The length of the text string 78 left you two columns with the size "UI" to display things like the scroll position of the buffer or line numbers.

100 lines is enough for someone, right?