What is the LaTeX convention for typing “xy-plane”?

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 1

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.)

Answer 2

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}

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):

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