Disclaimer: I'm no native speaker.
Thomas gets pronounced with a starting "T" (the "h" is silent), while Theodore with a "Th". What rule is followed here?
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There is no rule at all. As Robusto mentioned, there isn't really any rule for pronouncing th, and even if there were it's common for names not to follow rules.
Thomas comes from the Aramaic t’om’a, while Theodore comes from the Greek Θεόδωρος (Theodōros), which is probably the reason for the difference in pronunciation. Eventhough the th in Thomas comes from the later Greek spelling, it's likely that the pronunciation remained from the original form.
Unfortunately, the rule is pretty much that you have to know how it it is pronounced for every single word in the English language that begins with th-. Especially for proper names.
Even then it won't help if you work with two women named Thalia, one of whom insists on her name being pronounced Talia and the other wants the lithpier version.
Oh, and by the way ... Theodore's nickname is Ted! And you thought English pronunciations ought to make sense. That's just ... adorable! :)
P.S. I'm not poking fun at you, but at our silly, unreasonable language.
From the great poem on English pronunciation The Chaos:
The th will surely trouble you
More than r, ch or w.
Say then these phonetic gems:
Thomas, thyme, Theresa, Thames.
Thompson, Chatham, Waltham, Streatham,
There are more but I forget 'em-
Wait! I've got it: Anthony,
Lighten your anxiety.
It was actually written by a Dutch teacher of English.
In Australia and the US over the last 30 years, the written language seems to have trumped traditional UK pronunciation. Standard Aus. and US, in my experience, is 'th' fricative, like the 'th' in 'that'. Older UK speakers, on the other hand, seem to be sticking with a clear 'T' plosive pronunciation. Note that shorter version used sometimes to be written 'Thom' but always, to my knowledge, pronounced 'Tom', with a clear plosive 'T'.