Why is the default value of fill-column 70?

Question

Is there a reason that the default value of fill-column is 70?

In my experience, everyone who cares about the value of fill-column prefers a default value of 80.

Answer 1

This may have been intended for plain text emails, which get quoted in replies by adding a character (usually >) and often a space at the beginning of each line. 70 columns allow 8 levels of quoting on an 80-column terminal (8 > plus a space plus the rightmost column is reserved for a continuation character to indicate that the line overflows). Back in the days of physical terminals, 80 columns was a de facto standard.

Dominique wrote:
> Charlie wrote:
>> Bob wrote:
>>> Alice wrote:
>>>> I'm first!
>>> Second.
>> Third.
> Fourth.

Fifth.

-- 
Eve

I don't know for sure, however. I don't have a reference. It may well be that the column width dates back from before this email convention. But 80 columns was a very common terminal width even before Emacs existed, so 70 can't come from that alone.

The GNU coding standards requests a width of 79 characters (that's relevant to fill-column for string literals and comments), but that was added much later than the choice of the default value of fill-column in Emacs.