# How do you make a complement symbol?

I've often seen a notation where there is a horizontal bar over a letter to indicate the inverted value of something, e.g. q and (not)q for the outputs of a flip-flop. The not q would be a q with a bar over it.

More recently I was trying to describe the inputs to a 16 bit ADC, where one of the lines was R/(not)C.

I just waded through page after page of Unicode but could not find Roman letters with bars over them, except a few vowels.

Is there a way to do that in a Stack Exchange post?

You mean like $\overline{this}$?

Just invoke the \overline command in MathJax:

You mean like $\overline{this}$?


To have it not italicized you can use \text{} like in $\overline{\text{this}}$.

$\overline{\text{this}}$

• Cool. That's exactly what I mean. (voted.) Will that work on other Stack Exchange sites like Stack Overflow or the Arduino SO Beta? What kind of markup is that? – Duncan C Sep 1 '14 at 18:56
• I think it's pretty universal across all SE sites. It's called "MathJax", and it's based on LaTeX. There's plenty of documentation in the SE help and on the web in general. – Dave Tweed Sep 1 '14 at 19:52
• @DaveTweed since the engine is quite heavy, I think it only works here, on math and perhaps physics – clabacchio Sep 1 '14 at 21:28
• meta.stackexchange.com/questions/128228/… – clabacchio Sep 1 '14 at 21:39
• @clabacchio: After reading through that thread, and a few others linked to it, it would seem that there's more than just a few SE sites that currently have MathJax enabled. I guess we'd need to talk to an SE employee to get a definitive list. – Dave Tweed Sep 1 '14 at 22:01
• I tried it and it doesn't work on the Arduino SE site. Pity. I posted a feature request on the Arduino Meta SE asking for a way to enter this notation somehow. MathJax seems like a decent option. – Duncan C Sep 2 '14 at 1:33
• It doesn't really matter all that much. There are many commonly-understood ways to denote an active-low signal, including /SIGNAL, SIGNAL- and SIGNAL_L. Just pick one and go with it. – Dave Tweed Sep 2 '14 at 3:47
• There's a community maintained list of sites with MathJax support here: meta.stackexchange.com/questions/216606/… – PeterJ Sep 2 '14 at 10:31
• @DuncanC - It greatly reduces pageload speed, so unless there is a major need they try to avoid it. Just for a not in logic would not be reasonable. – Kortuk Sep 2 '14 at 15:51
• I've added the non-italicized version, I hope you don't mind ;) – clabacchio Oct 8 '14 at 8:03