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We can use \fontdimenX to modify the font parameters, where X can be 1 to 22. But how to modify the extra font parameters such as default rule thickness? Which is listed in Appendix G in TeXBook, Page 447.

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The list is also

What do different \fontdimen<num> mean

The syntax is the same for all of them, just need to use the suitable font (\textfont2 or \textfont3). I give a plain TeX example below, in latex it is the same but the timing needs to be synchronsied with the latex font setup (for example every size change changes the font assigned to \textfont3)

so rule thickness would be

enter image description here

\fontdimen8 \textfont3  =5cm

$ a \over b$

\bye

As Barbara comments below, this fontdimen affects the space above and below the rule as well as the rule itself, which is mostly unfortunate and usually constrains the amount of variation it makes sense to have here.

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  • you would have to choose the one that has the worst side effects. (of course, this is what is requested.) maybe you could provide a brief explanation of why this one is an example of unfortunate design. (one of the few really terrible flaws in the design.) Jun 2 '16 at 15:14
  • @barbarabeeton are you saying the AMS would not publish my document in this form? Shame. Jun 2 '16 at 18:22
  • I'm a bit confused. In my mind \fontdimen8 refers to s8(num1), and I want to access x8(default-rule-thickness). Are these two parameter joint or they have some relation?
    – Ben
    Jun 2 '16 at 19:10
  • BTW, my real goal is to shorten the minimal space limitation between superscript and subscript. Reading appendix G of the TeXBook, I think modify x8 is the only way to do this. (I've verified that the space in my document is limited by >=4{x8}, not any other parameters.) I'd appreciate it if there's other ways to achieve my goal without side effects.
    – Ben
    Jun 2 '16 at 19:18
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    @Ben fontdimen isn't a global value it is a per font parameter \fontdimen8 isn't usable as a length it parses for a font first. so \fontdimen8 \textfont2 is s8 \fontdimen8 \textfont3 is x8 Jun 2 '16 at 19:23

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