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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


I was wondering what goes wrong?

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marked as duplicate by yo', Mico, Kurt, diabonas, Jubobs Apr 30 '13 at 14:36

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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. – Jubobs 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
\mathcal{T} will give you a capital tau that doesn't just look like a regular T. – Alexey Kuleshevich Apr 26 at 7:55
up vote 19 down vote accepted

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.

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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...

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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
@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
@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

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.

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A good read when-not-to-use-ensuremath-for-math-macro – percusse Apr 30 '13 at 14:02
OK. If you know what you're doing there's no problem. When you don't there might be trouble. – Crowley May 1 '13 at 16:28

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