Ancient Greek nouns are cassified into three declensions, and we can say that this is largely based on the ending of the stem of the noun. If a noun's stem ends in (or in Attic when not after r, i, e), for example, then it belongs to the first declension; if it ends in -es, however, it belong to the third declension then. This may not be a perfect correlation, but this is not what is important here.

My question: Ancient Greek had diphthongs like ai, ei, au, eu. While there are nouns whose stem ends in diphthongs, the only permissible set of diphthongs in this context seems to be those ending in -u. We have basileus (βασιλεύς, king) whose stem ends in eu, and two singletons bous (βοῦς, cow, ox) and graus (γραῦς old woman) whose stems end in the diphthongs ou and au.

But why is there no noun whose stem ends in an -i-ending diphthong, like ai, ei, oi, ui, and their long versions? Is there an explanation based on historical linguistics, or is it really just a matter of unbelievably purest luck?

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    I'm not familar with Ancient Greek, but the way you present the concept of "stems" in this post seems a bit confusing. In the first paragraph, you talk about stems ending in "-o" and "-es". But in the second paragraph, you say that a word ending in "eus" has a stem ending in "eu". Why do you consider the final sigma to be part of the stem in nouns ending in "es", but not in those ending in "os" or "eus"? That is, are you asking about words ending in "-ai," or "-ais", or either? Commented May 29, 2017 at 5:45
  • (My impression from reading things about Latin was that the "stem" of an Indo-European noun doesn't include either inflectional endings, or thematic vowels, so the "o" of the second declension wouldn't be part of the stem.) Commented May 29, 2017 at 5:46
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    Sorry for not clarifying. The -es ending stems lose their -s- since it is between vowels (intervocalically). By "-es-ending stems" I actually mean words like Sōkratēs (Σωκράτης); the stem of this noun ends in -εσ, it's just the σ drops out when between vowels. So we have e.g. the genitive singular Sōkratous < Sōkrate-os < Sōkrates-os, where e-o regularly contract to ou in Attic Greek. Basileus, on the other hand, has the s just as a nominative singular suffix: g.sg. basileōs < basilē-os < basilēu-os; the -eu is nearly always -ēu, which is kind of confusing. Commented May 29, 2017 at 6:11
  • Also I changed the example from second declension to first declension because I was not too sure if second declension has -o or -e, if anyone reading is wondering. The vocative singular, supposedly bare, has -e, but everything else has -o. This is actually exactly like the situation of -eu nouns (-eu in voc. sg., -ēu everywhere else), except here most people and books think it is -o stem. Commented May 29, 2017 at 6:20
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    @sumelic. In standard parlance, the thematic vowel, if present, counts as part of the stem, but not of the root.
    – fdb
    Commented May 29, 2017 at 8:50

2 Answers 2


Only in a few cases you can find a meaningful answer to a question asking "why there is something lacking in a language". Normally, there is simply no answer. For example, why did not Ancient Greek have e Locative case in its nominal declension? Probably because it coalesced with the Dative. But why didn't it have a Comitative case? There is no answer to this question, because in Proto-Indo-European there has never been a Comitative case, without any reason.

Let's see whether type your question belongs to.

Usually, in historical linguistics to explain some feature of a language amounts to find its origin in some proto-language. So, let's start from clarifying what we mean by "stem" in the IE languages. The average IE noun can be either a radical noun (where case endings are attached directly to the root), or a thematic noun (where between the root and the case endings intervenes the so-called thematic vowel), or a noun that has a stem proper, i.e. a root plus a derivative suffix, with case endings attached directly to this stem. Now, let's exclude radical nouns and thematic declension and concentrate on nouns with a stem. Then your question must be reformulated: "why is there no nominal derivative suffix having the form of, or ending in, an -i diphthong?".

You are mentioning stems in -εύς as an example of diphthong ending stems. Now, let's exclude such words as βασιλεύς, Ἀχιλλεύς and other non-IE nouns whose origin is simply unknown and concentrate on words indicating professions: κεραμεύς 'potter' or ἁλιεύς 'sailor, fisher'. These stems are built with the suffix *ēw, which eventually evolved into something that was spelled, in Classical Greek, with the so-called diphthong ευ. But what is a diphthong? Phonologically a diphthong is hardly distinguishable from a mere sequence of a vowel plus a sonorant. In the teaching of Greek the notion of diphthong is widely used for the seek of simplicity, but it's more about spelling than about language. Suffice it to say, the "diphthongs" ου and ει are often just conventional spellings for long closed vowels /oː/ and /eː/ (outcomes of contraction).

All put together, you question can be reformulated in the following manner:

Why was there no PIE nominal suffix of the type -Vy- (where "V" indicates a vowel and "y" indicates a palatal sonorant), which then would evolve into something that is conventionally called "diphthong" in the Classical Greek spelling tradition?

Now, let's remember the PIE vocalic alternations: roughly speaking, every vowel could change into a zero. In the case of the suffix we are looking for this would result into a variant form -i-. Therefore, what we really need is a declension type with a stem ending in -Vy- in full (i.e. non zero) grade, preserved as such down to the Classical period. Unfortunately, there is no such thing.

  • We have words with the zero grade of the suffix -Vy-, i.e. the -ι declension (πόλις). It does not show any "diphthong" (with the possible exception of some oblique case forms).
  • We also have some words exhibiting the full grade of the suffix -Vy- (from IE *-oi-) but they have mostly lost the sonorant element, so no "diphthong" is there in the spelling. What I mean here is the declension in -ω, i.e. such words as ἠχώ 'echo'. The original Proto-Greek suffix is reconstructed as -οj- but the palatal element has been lost almost everywhere except for the dative form: ἡχοῖ.

References: Chantraine, Morphologie historique du grecque, §§82–88.

  • A propos the last point, Smyth 279 indicates that such nouns earlier did have the predicted iota subscript (Σαπφῴ).
    – user6726
    Commented May 29, 2017 at 16:23
  • @user6726 I am afraid Smyth is not a reliable source for studying the history of Greek seriously. The iota subscript is just an editorial device invented in the Late Antiquity and reflecting an attempt to graphically restore a sound that was already disappeared from the pronunciation by the end of the Classical Period. In some dialects the Nominative of the -ω declension had a final iota (written normally: -ωι), but, according to Chantraine (§88, footnote), with reference to Buck, Greek Dialects, this was an analogical creation on the basis of the Vocative -οῖ. Commented May 29, 2017 at 18:31
  • In inscriptions of the classical period the long diphthongs are written as ΩΙ, ΗΙ, ΑΙ. But of course not in the name Sappho.
    – fdb
    Commented May 30, 2017 at 23:35

τρεῖς “three” has the full-grade form of an IE -i stem, *trey-es.

  • Pedantically speaking, that is not a noun
    – Darkgamma
    Commented May 30, 2017 at 23:29

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