Using `\vspace` at end of paragraph creates blank new line

Question

\documentclass[11pt]{article} % Document class
\newcommand{\zzitem}[2]{%
\begin{description}
    \item[\textbf{#1}]: #2
    \vspace{-.75em}
\end{description}
}       

\pagestyle{empty}

\begin{document}
    \zzitem{Relevant Courses}{Calculus III, AP Computer Science, Business Statistics, Financial Accounting}
    \zzitem{Languages}{Russian, fluent; Spanish, conversational}
\end{document}

Here is the output of the above LaTeX:

However, if I remove just one letter on the first line, this is what I get:

This is my desired output, but if I were to add in that letter. What is causing this behavior to create a new line, but to not push anything onto this new line?

Answer 1

There is a space between #1 and \vspace (caused by the newline). So the offending paragraph ends with “Accounting<space><whatsit>”. The “whatsit” is an invisible object placed in the paragraph that will add vertical space after the line it happens to eventually be. You're unlucky, but you're the cause of it.

A blank line before \vspace{-0.75em} would do (or \par after #2, which would be exactly the same).

But is there any need for a single item description if you want to remove the space it produces?

\documentclass[11pt]{article}
\newcommand{\zzitem}[2]{%
  \par
  \addvspace{\topsep}%
  \noindent
  \hangindent=\leftmargini
  \textbf{#1}: #2\par
  \addvspace{\topsep}%
}
\pagestyle{empty}
\begin{document}
\zzitem{Relevant Courses}{Calculus III, AP Computer Science, 
  Business Statistics, Financial Accounting}
\zzitem{Languages}{Russian, fluent; Spanish, conversational}
Just for comparison, here's what you get with a \emph{single} description
\begin{description}
\item[Relevant Courses\normalfont:] Calculus III, AP Computer Science, 
  Business Statistics, Financial Accounting
\item[Languages\normalfont:] Russian, fluent; Spanish, conversational
\end{description}
\end{document}

Answer 2

Putting aside

• the supplied code does not produce the indicated output (I had to fix the comment on the first line and adjust the line width with \addtolength{\textwidth}{1.6in} to get the expected result).
• the approach to formatting is questionable

There remains the problem of understanding why an extra space would cause a new line to be created. After all, this doesn't happen normally in a LaTeX document.

And the reason for this is that \vspace doesn't do what a lot of people think it does. It does not start a new line and insert the space. Instead, it inserts the space after the current line if it appears in the middle of a paragraph. You can see this for yourself by running the following through LaTeX¹:

A surging, seething, murmuring crowd of beings 
that are human only in name, for to the eye and ear 
they seem naught but savage creatures, animated by 
vile passions and by the lust of vengeance and of hate.
\vspace{1in}
The hour, some little time before sunset, and the place, 
the West Barricade, at the very spot where, a decade 
later, a proud tyrant raised an undying monument to the 
nation’s glory and his own vanity.

\vspace is clever in that it will give the correct spacing regardless of whether there's a space before it or after it or both as is the case in this paragraph. You can try putting a % after hate. or after {1in} and you'll see identical output, but if you put in both %s, you'll get no space at all. But that cleverness comes at a cost. If it's at the end of a paragraph, the space that's normally discarded there will be retained.

egreg, surprisingly, managed to get the cause of the problem wrong. It's not that there's a space followed by a whatsit². In fact what's happening is that the code that does the space preservation in the \vspace command is responsible for putting an explicit space in the horizontal list that would otherwise have been dropped. We would get the same result if we had any other command that used the same mechanism to avoid extraneous spaces from going into the output such as \label.³

So, the bottom line is, to get your correct spacing the “best” solution would be to either add a blank line before \vspace so it's not treated as part of the paragraph or put a % after #2 so that there's no space to try to preserve. But I think I would instead, either use a description list as described in egreg's solution or, if you were committed to your structure, I would instead build up a single-item list environment with the skips set appropriately to format your output, but since that's beyond the question's scope, I won't offer an example here.

---

1. In the source code for LaTeX (texdoc source2e) you will see the following in the comments:

   [For example, is \vspace ever used in the middle of a paragraph?]

   the answer to this is, yes, but probably not intentionally!

2. As long as I'm being nitpicky, a \vadjust is not a whatsit but just vertical matter inserted into the main vertical list. Knuth had originally expected everyone to customize their TeX and add additional whatsits beyond the basic ones that are provided (\special and \write), but this never happened (except I think, but don't remember for sure that TeX-XeT's special codes for switching between L-R and R-L mode might have been implemented as whatsits).

3. I'll have to remember to add a warning about this in the section of my book where I talk about \label.