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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?

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In my opinion, "This question is not a good fit to our Q&A format... this question will likely solicit opinion, debate, arguments, polling, or extended discussion." Reworded, there may be many answers to this question, each of which depends on experience, and may vary ever so slightly from one another based on personal preference. –  Werner Dec 2 '11 at 16:34
This is totally a personal opinion, and therefore not an answer, but my criteria are: a) don't put the dash inside math, since it's not a minus sign but a dash, and b) Do put the xy inside math, since those are variable names. If a point were called (s,t) you'd call it the $st$-plane instead. So I'd go with your (3), which is what I do. –  Ryan Reich Dec 2 '11 at 16:36
One more variant is missing: XY-plane or \textsc{xy}-plane. It emphasizes that X and Y are dimensions, not coordinate variables. –  Andrey Vihrov Dec 2 '11 at 16:50
@Werner: I thought about it. (That is, I read the faq before posting.) But, it is hard to believe there is no convention. If you look at any textbook, there is a convention. Therefore, I am not seeking opinions here. –  Sony Dec 2 '11 at 17:01
@Sony: If every textbook has [the same] convention, then that is what you should use. I'm not claiming to be any expert; just giving my opinion based on what I read. You could post an image of this convention and request users to reproduce it, since typesetting experience may distinguish between spacing and alignment from an image. @ egreg made some excellent suggestions regarding the consistency of use, including the addition of \nobreakdash. –  Werner Dec 2 '11 at 17:04
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2 Answers

up vote 23 down vote accepted

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


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


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


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


and use it as \xyplane{}. (I've used both variations, take your pick.)

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Great answer with smooth introduction of \newcommand –  bobobobo Dec 3 '11 at 0:00
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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 $y$:


enter image description here

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

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

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I like your well-reasoned idea. Is this notation commonly used? –  Sony Dec 2 '11 at 19:02
I have no idea how commonly used this notation is, but I have seen it -- that's why I know about it. It's certainly not my invention! –  Mico Dec 2 '11 at 19:03
I personally would prefer $x$\nobreakdash-$y$~plane. I also consider the en dash incorrect in "Bose-Einstein condensate". In "pages 40--52" and "categories A--Z" the en dash signifies a range, but in "Bose-Einstein condensate" and "x-y plane" it's just a normal concatenation of words. This is evident in the case of "Bose-Einstein condensate" — it refers to the two physicists Bose and Einstein, not a range of physicists starting with Bose end ending with Einstein. For "x-y plane" one might argue that there's a range of axes from x to y, but that argument sounds artificial to me. –  celtschk Dec 3 '11 at 11:17
@celtschk: Thanks for these comments. I guess I would not restrict use of the endash to pure ranges, as in 40..52 or A..Z. For me, a short dash, when not used as a hyphenation character at the end of a line, can be used to represent a simple joining of two items (say, when a married person joins two last names). The longer endash, in contrast, "joins" items that don't have such a direct connection. The xy plane, to come back to the original issue, can be "created" by taking linear combinations of points on the x and y axes; for me, that's more than a simple "joining" of two items. :-) –  Mico Dec 3 '11 at 11:44
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