When should cdot be placed in brackets

math-modespacing

I was writing a document where I wanted to write the unit Newton meter. My code initially looked like this:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
    \[M=10\,\text{N}\cdot\text{m}\]
\end{document}

which outputs

enter image description here

it is clearly visible that the spacing between "N", the cdot and "m" is too large — it doesn't quite look like it belongs together (which it does since it's one unit).

I then looked at the Wikipedia page for newton meter and they used the following code1:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
    \[M=10\,\text{N}{\cdot}\text{m}\]
\end{document}

it's the same except the \cdot is placed in curly brackets like this {\cdot}. It renders like this:

enter image description here

which looks a lot better in this situation.

My question basically is: Why does placing a \cdot in brackets reduce/remove the space around it? Should this be used in any situation other than multiplying units?

1 This is not the exact code that the Wikipedia article uses (for example they also placed the \text{N} in brackets and used \mathrm{m} instead of \text{m}) but it gives the same result.

Best Answer

You see the same with 1+2 and 1{+}2

An expression surrounded with {} is a "\mathord" so an ordinary symbol that is given no additional space by TeX. Conversely by default + (and \cdot) is a \mathbin by default: that is, a binary operator and gets a medium math space added either side if used in infix position between two mathord symbols.

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