# [OS X TeX] Default rule thickness

Wed Aug 6 06:59:45 EDT 2008

On Wed, Aug 6, 2008 at 12:21 PM, Bruno Voisin <bvoisin at me.com> wrote:
> Le 6 août 08 à 09:34, Oliver Buerschaper a écrit :
>
>> 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.
>
> 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. Being a mere mortal, I must admit I can't make much sense of this
> appendix.
>
> 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>

The rule is between 0.4pt and 0.5pt

\starttext
\dontleavehmode
\scale[factor=200]{$1 \over 2$}
\scale[factor=200]{$1 \above .47pt 2$}
\stoptext

> 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}.
>
> I don't know what can be done for square roots.
>
> Bruno Voisin
>
> 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.

Wolfgang