Q:

Hyphenating/capitalizing values and coordinates

I'm translating some software from German (where there's a correct way for everything) into English (my native language, but also where I do so much more just by feeling) and I'm stumped by things like:

T value
  X coordinate
  Y coordinate

My sense is that all three should not be hyphenated or capitalized (though I'm not sure about at the beginning of a sentence.)  I should have made it more clear in writing the original question that I'm talking about capitalizing the words 'value' or 'coordinate.'
Some examples (not real, but of the sort I'd have to translate) might be:

Transform T value
X value out of range
Enter Y coordinate

Just recently the topic came up with my boss and she asked if that was the rule in English and I truthfully told her I didn't know. Is it the rule? Is there a rule?

A:

This isn't German; there is no rule. This isn't covered by English grammar, but by the conventions of whatever field of science/engineering you are writing in.
In the fields of math, physics, and computer science, the ones I am familiar with, the question is whether the variable you are talking about is named x or X. 
If it's named x, you never capitalize it, not even at the beginning of a sentence. And you try not to start a sentence with it. 
If it's named X, you always capitalize it (and you still try not to start a sentence with it). 
From Google Ngrams, it looks like people use both upper- and lowercase letters for all three of these quantities, although t-value is usually lowercase. 

