\vspace command inserting more space than specified

Question

I'm new to LaTeX altogether and discovered something strange while fiddling around with a few commands. Take this simple case of the vspace command:

\documentclass[]{article}

\begin{document}

\noindent
p

\vspace{10 mm}

\noindent
b

\end{document}

This should just insert a 10mm gap between the lower tip of p and the upper tip of b. Unfortunately, when I typeset it and measured the gap with a rule, it turned out to be around 18-19mm. I use TeXstudio as my LaTeX editor and was wondering whether the editor has something to do with this?

Answer 1

TeX doesn't work as you're conjecturing.

The normal distance between the two lines would be, measured from the baseline where the p is sitting to the next baseline (the b), 12pt.

The lower tip of the p is 1.94444pt below the baseline, the upper tip of the b is 6.94444pt above the baseline. So the normal distance between the two points is 3.11112pt = 1.09343mm

If you add to this 10mm you get a distance of 11.09343mm, which corresponds to the image below, generated by the minimal document

\documentclass{article}
\begin{document}

\noindent p

\vspace{10mm}

\noindent b%
\settoheight{\dimen0}{b}\smash{\raisebox{\dimen0}{\vrule height 11.09343mm}}

\end{document}

The line under \noindent b just adds the measuring rule that doesn't contribute to the normal spacing because of the \smash. If you get 18mm then you have other settings that you're not specifying.

Answer 2

To see what TeX is doing add

\showoutput
\showboxdepth=3

To your preamble, and TeX will log:

...\glue(\topskip) 5.69446
...\hbox(4.30554+1.94444)x345.0, glue set 339.44443fil []
...\glue 28.45274
...\glue 0.0
...\glue(\parskip) 0.0 plus 1.0
...\glue(\baselineskip) 3.11111
...\hbox(6.94444+0.0)x345.0, glue set 339.44443fil []

The two hboxes are your p and b, between them is the glue you specified, the baselineskip and parskip glue (which can stretch but won't in this case).

The vspace you added is additional space not the space between the letters. If you had not added a space then the space would not have been 0, TeX does not set lines of text so that ascenders of one line touch the descenders of the line above unless you set very non-standard settings of its parameters.