Height of subscripts

Question

I often have to deal with subscripts that look "too high". The most known example occurs when writing things like $\chi_\Delta$ but even with regular letters I find myself fixing what I feel like a problem.

Here is what I mean: consider

\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ $f_{k,j}\ \ \ f_{k,j}^{\vphantom{k}}\ \ \ \ a_{k,j}\ \ \ \ a_{k,j}^{\vphantom{k}}$
[
f_{k,j}\ \ \ f_{k,j}^{\vphantom{k}}\ \ \ \ a_{k,j}\ \ \ \ a_{k,j}^{\vphantom{k}}
]

I often find myself using the second and fourth options. There are answers here that address this (via a hack, it seems) and here that address this.

My question is, more than for a solution to avoid my vphantom, what is the logic within TeX that puts subscripts in the position they appear (as in the first and third cases above).

you are using inline math so tex is using a compressed style that tries to keep the expression within the standard line spacing of the paragraph. If you used display math tex would give itself more vertical space. — David Carlisle, Jan 26 '18 at 01:41
Yes actually not so much difference here but the main thing that I was trying (not so well) to say is the reason for the layout is to not disturb the line spacing, compare the first column with the second using your forced lowered subscripts here but sometimes it does work to change the defaults, you don't need to add phantoms everywhere, just change the fontdimens fintdiment16 mostly I guess, see https://tex.stackexchange.com/questions/88991/what-do-different-fontdimennum-mean/88993#88993 — David Carlisle, Jan 26 '18 at 02:03

score 3 · Accepted Answer · answered Feb 07 '18 at 22:25

David Carlisle has given the right hints in his comments but he has not written an answer. So I try to formulate a complete answer: Change the value of \fontdimen16 for \textfont2. It should get the value that \fontdimen17 has.

Here are some executable scenarios in plain TeX; they can be executed standalone:

% the current situation
\centerline{$
f_{k,j} \quad f_{k,j}^{\vphantom{k}} \qquad a_{k,j} \quad a_{k,j}^{\vphantom{k}}
$}
$$
f_{k,j} \quad f_{k,j}^{\vphantom{k}} \qquad a_{k,j} \quad a_{k,j}^{\vphantom{k}}
$$

% change fontdimen 16; note: this is always a global change
\fontdimen16\textfont2=\fontdimen17\textfont2
\centerline{$
f_{k,j} \quad f_{k,j}^{\vphantom{k}} \qquad a_{k,j} \quad a_{k,j}^{\vphantom{k}}
$}
$$
f_{k,j} \quad f_{k,j}^{\vphantom{k}} \qquad a_{k,j} \quad a_{k,j}^{\vphantom{k}}
$$

% increase the fontdimens 16 and 17
\fontdimen17\textfont2=1.25\fontdimen17\textfont2 % 1.25 is a guess
\fontdimen16\textfont2=\fontdimen17\textfont2
\centerline{$
f_{k,j} \quad f_{k,j}^{\vphantom{k}} \qquad a_{k,j} \quad a_{k,j}^{\vphantom{k}}
$}
$$
f_{k,j} \quad f_{k,j}^{\vphantom{k}} \qquad a_{k,j} \quad a_{k,j}^{\vphantom{k}}
$$
\bye

I don't think it is a 'hack' to change the value of the \fontdimen16. The TeXbook, p. 179, contains the following text, which states more or less your problem:

Instead of changing the sizes of subformulas, or using |\raise|, you can also control vertical spacing by changing the parameters that \TeX\ uses when it is converting math lists to horizontal lists. These parameters are described in Appendix G; you need to be careful when changing them, because such changes are global (i.e., not local to groups). Here is an example of how such a change might be made: Suppose that you are designing a format for chemical typesetting, and that you expect to be setting a lot of formulas like $\rm Fe_2^{+2}Cr_2O_4$ . You may not like the fact that the subscript in $\rm Fe_2^{+2}$ is lower than the subscript in $\rm Cr_2$ ; and you don't want to force users to type monstrosities like

\begintt
$\rm Fe_2^{+2}Cr_2^{\vphantom{+2}}O_4^{\vphantom{+2}}$
\endtt

just to get the formula $\rm Fe_2^{+2}Cr_2^{\vphantom{+2}}O_4^{\vphantom{+2}}$ with all subscripts at the same level. Well, all you need to do is set '\fontdimen16\tensy=2.7pt|' and '|\fontdimen17\tensy=2.7pt|', assuming that |\tensy| is your main symbol font (|\textfont2|); this lowers all normal subscripts to a position $2.7\pt$ below the baseline, which is enough to make room for a possible superscript that contains a plus sign.

The details about the fontdimens are described in Appendix G of The TeXbook. A very nice TUGboat article by B. Jackowski explains the rules of this Appendix with diagrams; see figures 9--11 for the placement of subscripts.

Height of subscripts

1 Answers1