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


I was wondering what goes wrong?

  • 2
    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
    Commented Apr 30, 2013 at 13:44
  • But I need capital "t" to represent another different quantity.
    – Tim
    Commented Apr 30, 2013 at 13:48
  • Then use different font family for your "Tau"
    – Crowley
    Commented Apr 30, 2013 at 14:00
  • 4
    \mathcal{T} will give you a capital tau that doesn't just look like a regular T.
    – lehins
    Commented Apr 26, 2016 at 7:55

3 Answers 3


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:


for latex, or

\def\Tau{{\rm T}}

if you're using plain tex.


As barbara suggested


will typeset Roman T.

When loading

\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.


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...

  • 4
    Interesting how "they" didn't do the "semantic thing" in this particular case. After all, \Tau means something substantially different from 'T'. Commented Apr 30, 2013 at 13:52
  • 2
    @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. Commented Apr 30, 2013 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'
    Commented Apr 30, 2013 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" Commented Apr 30, 2013 at 14:11
  • 1
    @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. Commented Apr 30, 2013 at 14:12

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