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These words share the Greek root πτέρυξ (pteryx), meaning feather/wing, but the P in pterodactyl is silent (in the initial position), while the P in archeopteryx (in the middle of the word) is voiced.

Why is this, and what's the rule? Is it just that that's how the ancient Greeks pronounced it, or is there something more subtle going on?

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NB: A voiced /p/ is a /b/. :) – tchrist Feb 5 '13 at 0:30
Or in case you don't quite get what @tchrist means, voiced has a specific meaning when talking about pronunciation, with both voiced and unvoiced being heard. As he says, /p/ and /b/ are a voiced-unvoiced pair. – Jon Hanna Feb 5 '13 at 0:34
Darn. I knew that scrimping on my studies of Devanagiri would bite me some time... – James McLeod Feb 5 '13 at 3:02
Oh ptooey. It's hard to pick a best answer. +1 for everyone. I'm going to accept Jon Hanna's answer since he was first. – James McLeod Feb 7 '13 at 1:13
up vote 9 down vote accepted

Π (pi) in Greek is pronounced as P in English, and Τ (tau) in Greek is pronounced as T in English.

Greek though is quite okay with PT at the start of a syllable, while that isn't a phoneme cluster we have in English.

So, while we can pronounce both letters if we end one syllable with P and start the next with T, we will make the P silent at the start of a word.

This has happened for some time, since while pterodactyl is a relatively recent word, we've been talking about Ptolemy for quite some time.

Comparably, when we import Ξ from Greek as X, we'll pronounce it KS in the middle of a word, but change to a Z sound at the beginning, as discussed at this question.

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Er, you wrote "\N{N-ARY PRODUCT} (pi) in Greek is pronounced as P in English, and \N{GREEK CAPITAL LETTER TAU} (tau) in Greek is pronounced as T in English." That is, you used U+220F N-ARY PRODUCT, whereas what you really wanted is U+03A0 GREEK CAPITAL LETTER PI, written Π — which while remarkably similar to ∏ in most fonts, is a different code point altogether. – tchrist Feb 5 '13 at 0:41
Thanks @tchrist As well as being what I did indeed want, it's nicer looking in the font I'm seeing it in, too. – Jon Hanna Feb 5 '13 at 0:44
@tchrist you wrote "Er" whereas what you really wanted is "psst". I would also explain that N-ARY PRODUCT is not a letter. Its appearance will vary in web browsers and in other media where this open content (CC-BY-SA) answer may appear. But I see that it has been changed to GREEK CAPITAL LETTER PI. – minopret Feb 5 '13 at 12:48
PT is not a phoneme in any language I've ever encountered. It's usually regarded as a cluster. – Colin Fine Feb 5 '13 at 13:30
Thanks @ColinFine that is correct. Just muddled thinking on my part. – Jon Hanna Feb 5 '13 at 13:41

The trick is to get the two stops into separate syllables. With archaeopteryx, the p ends one syllable and the t starts the next one. With pterodactyl, no such luxury is possible, so the p is lost.

See also apnea versus pneumonia.

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First, the English rule has to do with syllable structure. As Colin points out, English syllable can't begin with the consonant cluster /pt/. Any word spelled that way will be pronounced some other way.

But the sequence /pt/ can occur between syllables. So if the vowel before /pt/ is stressed (as in archeopteryx /ar.ki.'ap.tər.ɪks/), then both stops can be pronounced, each in its own syllable, as Jon Hanna has noted. That's the English rule.

Of course, both words are made using the Greek word πτέρυξ 'wing', and this word, as well as the English words feather (p -> f and t -> θ via Grimm's Law), compete, perpetual, ptomaine, symptom, and hippopotamus, all are descended from the PIE root *pet- 'to rush, fly'. The /p/ and the /t/ were both pronounced in Greek.

Finally, the English word helicopter is also transparently derived from helico- 'rotating, spiral' plus -pter- 'wing'. What's interesting about this is that helicopter is such a common word that it's been split into combining forms, but not the original ones.

In English, helicopter is divided into heli- (heliport, helipad, helitaxi, helitours, ...) and -copter (cargo copter, minicopter, jetcopter, ...), because the glue between helico- and -pter- wouldn't come off, but the glue between heli- and -copter turned out to be soluble in English phonology.

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Very interesting point about the pseudoprefix "heli." – James McLeod Feb 7 '13 at 1:14

No, it is not how the ancient Greeks pronounced it: /pt/ was a possible initial cluster (as were /pn/, /ps/ and /kt/).

In English, /ps/ is a possible initial cluster (though it is rare, and pretty well confined to words of Greek origin) but /pt/, /pn/ and /kt/ are not possible initially, and get simplified (as does /kn/, which was possible in Old English).

All these clusters are normal medially, and generally split the consonants between syllables. Thus uptake and archaeopteryx contain /pt/ medially.

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Probably because "pssst!" is possible. pssseudo, pssssylium, ... :) KN was possible in Old English? I did not kuh-now that! :) – Kaz Feb 5 '13 at 4:05
@Kaz as was GN, HL, HN, HR and HW. GN and HW often leave their mark on modern spelling (the G of GN silent, and HW turned into WH), while the others generally don't (so raven and lady have lost their R) – Jon Hanna Feb 5 '13 at 8:59
@Kaz sorry, lost their H is what I meant to say. – Jon Hanna Feb 5 '13 at 13:39

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