निरंजन
I was getting one error (i.e. `! Missing number, treated as zero.`) which I found irrelevant. I checked the brackets many a times and I think I am right at them. My question is why is `\\` good for the first line, but not good for the second line? Also replacing `\par` with a blank line gives the exact result that I want, but why `\\` doesn't work? The code is as follows -
```
\documentclass{article}
\setlength{\parindent}{0pt}
\begin{document}
[abc]\\
def\par%\\
[ghi]\\
\end{document}
```
Top Answer
Skillmon
Your issue has nothing to do with LaTeX's math typesetting or the material you put in math mode. A minimal non-working example to this looks like the following:
```tex
\documentclass{article}
\begin{document}
[abc]\\
[def]\\
[ghi]
\end{document}
```
The issue is that `\\` takes an optional argument, and while parsing for that argument spaces are ignored, so the above is equivalent (for LaTeX) to:
```tex
\documentclass{article}
\begin{document}
[abc]\\[def]\\[ghi]
\end{document}
```
What happens now is that `\\` takes the optional argument (`def` in the first and `ghi` in the second case) and tries to use that optional argument, which should specify an additional vertical space inserted after the linebreak, as a correct dimension. So TeX begins to parse `def` as a dimension, finds that this is not a valid skip or dimen register and parses for a valid float followed by a valid unit, hence throwing the missing number error.
To solve this you have to "hide" the `[` as the opening delimiter of the optional argument. You can do so by putting a `\relax` after `\\`:
```tex
\documentclass{article}
\begin{document}
[abc]\\\relax
[def]\\\relax
[ghi]
\end{document}
```