Why do I get a 'macro parameter # in vertical mode' error?

macros errors

1,755

The errors I get are

! Missing { inserted.
<to be read again> 
                   \vcenter 
l.12     \[ \textbelow{Look Down}{hello \\ world}
                                                  \]
? 
! Missing } inserted.
<inserted text> 
                }
l.12 ...[ \textbelow{Look Down}{hello \\ world} \]

which clearly stem from the missing braces around \substack{...}.

Indeed, if I add the braces, the example code runs without problems.

\documentclass{article}
\usepackage{amsmath}

\newcommand{\textbelow}[2]{\underbrace{#1}_{\substack{#2}}}

\begin{document}

This \(\underbrace{Look Down}_{\substack{hello \\ world}}\) works fine.

But the newcommand does not:
    \[ \textbelow{Look Down}{hello \\ world} \]

\end{document}

However, this doesn't seem good for typesetting text.

1,755

Nanashi No Gombe

Updated on August 01, 2022

Comments

Nanashi No Gombe over 1 year
I am trying to define a new command in the following manner:
```
\newcommand{\textbelow}[2]{\underbrace{#1}_\substack{#2}}
```
It looks like the underscore character _ is creating the problem. If I remove it, I do not get any error, but that is not what I want to do.

The syntax is correct, because outside of newcommand it works fine.
```
\[\underbrace{Look Down}_{\substack{hello \\ world}}\]
```
Why am I not able to make a new command out of this? If it is not possible to write an underscore _ inside newcommand, what is the workaround?

Thank you for your assistance.

If you so require, here is a full example of what I am trying to do.
```
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}

\newcommand{\textbelow}[2]{\underbrace{#1}_\substack{#2}}
\newcommand{\test}[2]{\underbrace{#1}\substack{#2}}

\begin{document}

This $\underbrace{Look Down}_{\substack{hello \\ world}}$ works fine.

But the newcommand does not:
    \[ \textbelow{Look Down}{hello \\ world} \]

\end{document}
```
Edit:

@egreg's solution does not work for me. Here is a screenshot of the error I get in TeXstudio.
- egreg over 5 years
  
  The syntax is incorrect: \newcommand{\textbelow}[2]{\underbrace{#1}_{\substack{#2}}} would be right.
- egreg over 5 years
  
  That example surely doesn't produce the error.
- Nanashi No Gombe over 5 years
  
  @egreg I guess not. But would you terribly mind if I asked you to make a guess at why I might be getting the error? Could it be because of how I have structured my document (separate folders for packages, macros, chapters, main tex file etc.)?
- egreg over 5 years
  
  Without a real example producing the error, it's impossible to say. Check your braces, not only in that definition.
- Nanashi No Gombe over 5 years
  
  @egreg I got it. Thanks for asking me to check the log file. ;)
Nanashi No Gombe over 5 years

Thank you for your prompt answer. Maybe there is something else I am not telling you, but I get an error even with your simple fix. Please check the screenshot I added to my question.
egreg over 5 years

The “macro parameter in vertical mode” error usually means you have misplaced braces. Rather than the screenshot, please add the real error message you find in the .log file.
Nanashi No Gombe over 5 years

I tried copying and pasting the exact code from your answer to the page I am currently working on. I still get the error.