Differentiating between \hbox and \mbox

Question

What are the differences between \hbox and \mbox?

Related:

• This answer to "What are the different kinds of boxes in (La)TeX?" discusses different boxes in general.
• This answer to "Difference between various methods for producing text in math mode" differentiates \text from \mbox.

Answer 1

\hbox is a TeX primitive and \mbox is a LaTeX macro defined via

\long\def\mbox#1{\leavevmode\hbox{#1}}

The \leavevmode means that it starts a paragraph, compare:

\hbox{one}
\hbox{two}
three

with

\mbox{one}
\mbox{two}
three

The other difference is that the argument is parsed as a normal macro argument, so for example \mbox a works and is identical to \mbox{a} whereas \hbox a is a syntax error. \mbox{\verb|{|} will lead to errors but \hbox{\verb|{|} will box a verbatim {.

If you do use a brace group for the argument it has to be explicit {} (or other characters with catcodes 1 and 2) but \hbox can use implicit braces such as \hbox\bgroup ... \egroup (in particular you can start a box in one macro and end it in another).

Don't use \hbox in a LaTeX document (although you will often see it in internal package code).

---

As requested, some further details on \leavevmode:

\documentclass{article}

\begin{document}

compare

\hbox{one}
\hbox{two}
three

with

\mbox{one}
\mbox{two}
three

see?


compare

zero
\hbox{one}
\hbox{two}
three

with

zero
\mbox{one}
\mbox{two}
three

see?

\end{document}

After the first compare is a paragraph break, so TeX goes into vertical mode. In vertical mode, boxes are stacked vertically. The next two boxes are \hboxes with one and two so they are appended vertically to the current vertical list. Then comes the letter t of three; letters are not allowed in vertical mode so TeX pushes it back into the input and instead starts a new paragraph, adds the paragraph indentation and then sees the three again in horizontal mode.

After the first with there is again a paragraph break, but this time TeX sees \leavevmode (or more exactly the \unhbox in its definition), so it starts a new paragraph, adds the indentation and then sees \hbox{one} in horizontal mode. \leavevmode does nothing in h-mode so after the glue from the word space resulting from the end-of-line character, TeX sees \hbox{two}, which comes horizontally after one, and three follows in the same paragraph.

After the second compare, TeX is again in vertical mode, but this time the paragraph is started by zero, so TeX is already in horizontal mode before it sees the \hbox, so \hbox and \mbox act the same way.

Basically LaTeX goes to some lengths to never expose the primitive TeX box behaviour: all LaTeX boxes start with \leavevmode so that they act like \mbox and not \hbox. (The difference is basically the same as the difference between \parbox (LaTeX) and \vbox (TeX) and \rule (LaTeX) and \hrule (TeX) for example.)