Question: I want to type "xy-plane" in a document so that it "looks right" when I print the document.

(1) `xy-plane` does not look right to me.  

(2) `$xy-$plane` looks bad.  

(3) `$xy$-plane` looks ok, but I feel uneasy. 

What is the accepted convention for typing "xy-plane" in a (La)TeX document?

Answer: 
Here "x" and "y" are variables, so they have to go in math mode; the hyphen must go outside math mode. So

```
$xy$\nobreakdash-plane

```

Note that `\nobreakdash` is provided by the **amsmath** package; it disallows a break after the hyphen. Possible variation:

```
$x\,y$\nobreakdash-plane

```

where the two letters are slightly separated. Be consistent. You can define

```
\newcommand{\plane}[2]{$#1#2$\nobreakdash-plane}

```

and use it as `\plane{x}{y}`, if it appears with different letters. Or

```
\newcommand{\xyplane}{$x\,y$\nobreakdash-plane}

```

and use it as `\xyplane{}`. (I've used both variations, take your pick.)
Here's a different suggestion: I wouldn't place a hyphen before the word "plane" but, instead, an unbreakable space. Second, I'd insert an (unbreakable) en-dash between $x$ and $y$:

```
\newcommand\xyplane{$x$\nobreakdash--$y$~plane}

```

![enter image description here](https://i.stack.imgur.com/q4dQj.png)

The en-dash is, typographically speaking, the most frequently used "connector" symbol, as in "pages 40--52", "Einstein--Bose condensates", "categories A--Z", etc. The notation implies, so to say, that we're talking about the plane formed by (linear combinations of) points on the $x$ and $y$ axes. 

I'd note, separately, that I wouldn't foresee much of a chance for confusion between $x$--$y$ and $x-y$, because TeX inserts whitespace (of width `\medmuskip`) around binary operators (such as the minus sign):

![enter image description here](https://i.stack.imgur.com/Syz7c.png)

Note also that the endash is *thinner* than the math-minus sign.