[TUHS] why the leading under score added to function names?

[arnold at skeeve.com]

Hi All.

Recently at work I helped someone figure out that when working with ld,
the name of a function "foo" gets turned into "_foo" by the compiler.
(It took this old-timer 15 minutes to solve a problem he had been working
on for two days!)

I'm pretty sure this dates back to PDP-11 days.  I'm wondering "why?".
Why did the C compiler prepend an underscore to function names?

Thanks,

Arnold