TeX question about double underlining in math mode

Question

I am trying to define a macro for double underline in TeX (not LaTeX).

The title and the body of your posting seem to contain some ambiguity: Is the double-underlining supposed to happen in text mode or math mode? — Mico, Jun 10 '15 at 16:24
Well, for example, what are you trying to underline? A single symbol? A very long math expression? — Alenanno, Jun 10 '15 at 16:24

score 19 · Answer 1 · answered Jun 10 '15 at 16:26

19

The first idea that comes to mind is

\def\doubleunderline#1{\underline{\underline{#1}}}

$\doubleunderline{a+b}$

\bye

enter image description here

answered Jun 10 '15 at 16:26

egreg

1

The \underline{\underline{#1}} has the "drawback" that the underlines of two consecutive underlined symbols merge, e.g. $\doubleunderline{a}\doubleunderline{b}$ , which is often not what one would want in the context of writing matrices double underlined... – Christoph90 Jan 08 '18 at 10:13
@Christoph90 Nobody should use double underlining to begin with. ;-) – egreg Jan 08 '18 at 12:31
This also has the drawback of letting an awkward space between the two lines. – Arnaud Mortier Jun 09 '18 at 17:52

1 Answers1