# [OS X TeX] Default rule thickness

Oliver Buerschaper oliver.buerschaper at mpq.mpg.de
Wed Aug 6 06:39:17 EDT 2008

>> does someone know how to find out the current value of
>> default_rule_thickness in a (La)TeX document? TeX uses it to
>> determine the line width of fraction bars and roots in math mode …
>
> The short story: 0.4 pt.
>
> The long story: LaTeX defines \frac in terms of the TeX primitive
> \over, and \sqrt in terms of the TeX primitive \radical (a primitive
> being a command defined in the source code of TeX itself). Hence the
> only way to get an answer for sure would be to inspect the source
> code of TeX.

That's rather intimidating …

> The TeXbook has an appendix G "Generating boxes from formulas"
> mentioning in several places (for \overline, \underline,
> \radical, ...) a default_rule_thickness called either $\theta$ or $> \xi_8$, but not giving its value.

This is exactly where I found the name … but not the value ;-)

> Being a mere mortal, I must admit I can't make much sense of this
> appendix.

Same for me …

> My feeling is that TeX uses the same default rule thickness for all
> rules (in \overline, \underline, \over, \sqrt, \footnote, ...) and
> that it is the thickness given at the beginning of chapter 21
> "Making Boxes":
>
> \hrule width=<depends on contest>, height=0.4pt, depth=0.0pt
> \vrule width=0.4pt, height=<depends on contest>, depth=<depends on
> contest>

I also stumbled upon this and I guess its a reasonable value to assume
for the standard font size.

But do you know how TeX (or LaTeX that is) scale this value with
increasing global font size?

> Now, if you want to specify the rule thickness yourself for a
> fraction, say to 1pt, I see two solutions:
>
> - In plain TeX, replace \over by \above. For example, replace {a
> \over b} by {a\above1pt b}.
>
> - In LaTeX, use the amsmath package and replace \frac by \genfrac.
> For example, replace \frac{a}{b} by \genfrac{}{}{1pt}{}{a}{b}.

Actually, I'm not going to change the thickness of these rules, I'd
rather like to draw lines of equal thickness for some sort of
graphical notation within displayed formulas …

Oliver

> PS Normally, such a question which has nothing Mac-specific would be
> better posted to a general LaTeX forum such as comp.tex.text.
> However, comp.tex.text may be quite intimidating to the novice
> (chances are you'll either get your question ignored or be flamed
> for asking a stupid question), and as a result I have myself asked
> general questions here more than once and avoid comp.tex.text
> whenever possible.

Perhaps you're right. I'll try and take the plunge then …