From: helbig@asclothestro.multivax.de   
      
   Some journals require upright Greek letters for constants as opposed to=20   
   variables, for example $\upi$ when used to denote 3.14159... as opposed=20   
   to a variable ($\pi$ is sometimes used to denote parallax in astronomy,=20   
   for instance).  (Some journals define \upi as "upright pi", \upi as=20   
   "upright i", and so on.)   
      
   I certainly agree that LABELS should be upright (though they are usually   
   Latin not Greek) and not italic to distinguish them from variables, e.g.   
   $T_{mathrm{eff}}$ for effective temperature or $\rho_{textrm{g}}$ for   
   gas density, say, as opposed to $G_{\mu\nu}$ where $\mu$ and $\nu$ are   
   not constants but variables.=20   
      
   And it is not just Greek letters.  For example, e for the Euler number=20   
   or i for the square root of -1 should also not be in math italic, to=20   
   distinguish them from variables.  I tend to agree with that as well. =20   
   Also, units should be upright, e.g. 5 m and not $5m$ for 5 metres.   
      
   On the other hand, I have never seen the gravitational constant $G$,=20   
   which is even by definition a constant and not a variable, written=20   
   upright.  Ditto for the Hubble constant $H$ and so on.=20   
      
   Or is there a difference between mathematical constants and physical=20   
   constants?   
      
   Perhaps because standard (La)TeX provides Greek letters only in math=20   
   italic, upright Greek letters are less common than upright Latin=20   
   letters, even when used in the same way (labels, units, symbols which=20   
   are not variables).   
      
   When writing for a specific journal, one usually has to follow the house=20   
   style.  However, if there is no rule, I prefer to do what is generally=20   
   deemed to be correct.  What is generally deemed to be correct here? =20   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)