No, I'm no expert on this one. It's like an underline, but written above
the characters on a line -- an overscore. It's not a macron (¯), because
it is continuous across several characters; it's plainly not the
continuing stroke that's so distinctive in Devanagari. It's likely that
many Qalamites know of it.

A 19th-century algebra text (by Olney) used this more often than modern
math. does; it serves to group several items together, parenthesis-style,
e.g. (a+b)/c. It survives in our modern square root symbol* as the
horizontal stroke at upper right. Pencil-and-paper computation of square
roots (and long division, as (was?) taught in US schools, at least) still
uses it as an overscore, but not with the combining meaning.
*Historically, more like a check mark

Formal logic (math), at least traditionally, used it to combine, as well
as to help describe the relationship in an expression (try "Sheffer
stroke"). Such logic expessions are at the heart of computer design, and
while I was an editor ('77-'79) at Electronic Design magazine, we heard
periodic howls of pain from our typographer when we sent copy that used
it. (We had manual typewriters, and drew our overbars by hand with a
ruler.)

Apparently, their typesetter* had no provision for creating an overbar. (I
wouldn't be surprised if modern computer typography generally can't do it,
either. Eric Weisstein's superb Web math site didn't have an example
under "NAND"; I was *very* surprised.)
*Guessing, but maybe spinning photo font wheels, lenses, and flash lamps?

Nevertheless, our typographers did what they had to; I never learned any
details of how they did it. It could have been as ugly as cutting and
inserting bits of film; I hope not.

It seems that life was much easier back in the days of "cold" type, before
Linotype. An overbar could simply be a piece of rule, cut and positioned
properly, with leading on (usually) both ends, it seems to me.

Trying a sequence of macrons: ¯¯¯¯ Got a nice, thin (1 px?) continuous
line, there. (Using Verdana 14, bold, to compose e-mail. Love it.)

--
Nicholas Bodley /*|*\ Waltham, Mass. (Not "MA"; "Ma" is mother...)
Olney told how, in detail, to calculate cube, fifth, and seventh roots
by pencil and paper. Fifth and up were a lot of guesswork and
cumbersome trial (and error, if you guessed wrong) Explained theory, too.