\vspace vs. \vskip

Question

What is the difference between \vspace{-1em} and \vskip -1em, for example? I guess the first is LaTeX, and the latter is TeX. When is the proper time to use one and not the other, and why?

Answer 1

At any point in its processing TeX is in some mode. There are six modes, divided in three categories:

1. horizontal mode and restricted horizontal mode,
2. vertical mode and internal vertical mode, and
3. math mode and display math mode.

When not typesetting mathematics, TeX is in horizontal or vertical mode. Horizontal mode is typically used to make lines of text; vertical mode is typically used to stack the lines of a paragraph on top of each other.

\vskip inserts a glue in a vertical list of the lines. Therefore \vskip breaks the horizontal mode and goes to the vertical mode.

\vspace can work in horizontal mode and vertical mode. In horizontal mode \vspace 1mm is equivalent to \vadjust{\vskip 1mm \vskip 0pt} and inserts a space after the current line. In vertical mode \vspace 1mm is equivalent to \vskip 1mm \vskip 0pt

\vskip 0pt is needed to \removelastskip could not remove your vertical space.

Full definition:

\vspace : -> \@ifstar \@vspacer \@vspace 

Without * case (\vspace 1mm)

\@vspace: #1->\ifvmode \vskip #1 \vskip \z@skip \else \@bsphack \vadjust {\@restore
par \vskip #1 \vskip \z@skip }\@esphack \fi 

Note \z@skip equal to 0pt, \@bsphack is needed to save big horizontal space after period. \@esphack is needed is needed to return big space settings.

With * case (\vspace* 1mm)

\@vspacer:#1->\ifvmode \dimen@ \prevdepth \hrule \@height \z@ \nobreak \vskip #1 \v
skip \z@skip \prevdepth \dimen@ \else \@bsphack \vadjust {\@restorepar \hrule \
@height \z@ \nobreak \vskip #1 \vskip \z@skip }\@esphack \fi 

Answer 2

\vspace is a LaTeX command and \vskip is a TeX command. Both can be used in LaTeX. When \vspace is used within a paragraph, it inserts space after the current line. But if \vskip is used within a paragraph it ends the paragraph and inserts the space immediately.

Answer 3

Exactly as Rob says; to rephrase, \vskip should be used when you are in, or want to start setting in, vertical mode, and \vspace makes sense within a paragraph.

The (an?) implementation of \vspace is discussed in Eijkhout 1990, TUGboat 11(4), p618.