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I would like to typeset the capital Greek letter for tau. I used \Tau, but I got error for that:

Undefined control sequence.

My preamble is 

\documentclass[10pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

I was wondering what goes wrong?

Count Zero
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Tim
  • 5,763
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    The capital "tau" is nothing other than a capital "t"; see this. That's why TeX does not define a control sequence associated for capital "tau". Similarly, capital "alpha" is "A", capital "beta" is "B", etc. – jub0bs Apr 30 '13 at 13:44
  • But I need capital "t" to represent another different quantity. – Tim Apr 30 '13 at 13:48
  • Then use different font family for your "Tau" – Crowley Apr 30 '13 at 14:00
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    \mathcal{T} will give you a capital tau that doesn't just look like a regular T. – lehins Apr 26 '16 at 07:55

3 Answers3

42

since many of the greek uppercase letters have the same shape as latin letters, they were not separately provided for tex, for two reasons:

  • to save space (which was a real consideration in 1980);

  • because it wouldn't be possible to tell the difference between the two shapes.

the first reason is no longer relevant, of course, but the second is.

go ahead and define your own:

\newcommand{\Tau}{\mathrm{T}}

for latex, or

\def\Tau{{\rm T}}

if you're using plain tex.

egreg
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barbara beeton
  • 88,848
10

As barbara suggested

\newcommand\Tau{\mathrm{T}}

will typeset Roman T.

When loading

\usepackage{amssymb}
\newcommand\Tau{\mathcal{T}}% Caligraphic T for example

You can access more symbols for your purpose. At Detexify you can find most common symbols and identify them from handwritten image.

Crowley
  • 4,297
2

Nothing goes wrong. :)

Quoting from the The Not So Short Introduction to LATEX2e (ver 5.01) - p. 56

There is no uppercase Alpha, Beta etc. defined in LATEX2e because it looks the same as a normal roman A, B...

Count Zero
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    Interesting how "they" didn't do the "semantic thing" in this particular case. After all, \Tau means something substantially different from 'T'. – Brent.Longborough Apr 30 '13 at 13:52
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    @Brent.Longborough: What about the capital Cyrillic Te (written "T") - is it the same as the capital Greek tau which it is based on? I'm no expert on lettering, but it seems unremarkable to me that different alphabets can share the same glyphs. – Charles Stewart Apr 30 '13 at 14:01
  • @Brent.Longborough however, these two letters use the same symbol and in the same way (\mathord). Which means that they are identical for LaTeX. And not only for LaTeX, they are identical for a mathematician as well. For linguists, they are different, but you don't use $\Tau$ as a lingust, right? :) – yo' Apr 30 '13 at 14:10
  • @CharlesStewart Oh, yes, the glyphs are the same, but the semantic context is different, and we are always told (correctly, IMO) to "do LaTeX things semantically" – Brent.Longborough Apr 30 '13 at 14:11
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    @Brent.Longborough barbara described the shortage of space in the early days of tex; latex2e had a similar experience (remember, it appeared when many people were using rather small 286-based machines) -- there are lots of oddities about the kernel's coding, many of them simply to save csnames. in that circumstance, there's no surprise that there's no "semantic" \Tau dating from the early days, and nowadays people who care about these things are using entirely different set-ups, so there's little incentive now, either. – wasteofspace Apr 30 '13 at 14:12
  • @Brent.Longborough - There is some sense to what you say, e.g., Unicode represents T as different code points in its Latin, Greek, and Cyrillic code tables. Though note that if you use Luatex or Xetex, you have the Unicode ontology as part of Latex: just choose a maths font with enough characters and use the Unicode in your Latex source file. And I'm not sure there's a real problem: Unicode's identical glyphs for different code points is often a nuisance. – Charles Stewart May 01 '13 at 11:58
  • In a sense, my previous comments, were, I admit, "mild trolling". Some of @CharlesStewart 's "nuisance" is, occasionally, necessary: for example, for \MakeLowerCase{} to behave correctly. – Brent.Longborough May 01 '13 at 12:57
  • @CharlesStewart -- the unicode "identical glyphs ... nuisance" does have some positive side effects, including the ability to correctly alphabetize the base language for the script without jumping through hoops. – barbara beeton May 30 '13 at 13:49