# [OS X TeX] Default rule thickness

Bruno Voisin bvoisin at me.com
Wed Aug 6 06:21:24 EDT 2008

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>

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.