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Foedus is both a noun and an adjective. As a noun, it is classified as third declension, since its genitive ends in -is: foederis.

I am intrigued however on the added "er" syllable. Why not foedis? Other cases (but not all of them) also have the "er" syllable added.

My guess is that this addition is to differentiate between the noun and the adjective. Compare:

Adjective - Noun

foedis - foederis
foede - foedere
foedum - foederum
foeda - foedera
foedi - foederi

The second column, without the extra "er" syllable, would be just as the adjective. Is this the reason of the added "er"? Is this common in Latin?

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    Nothing was “added” in the oblique cases. s > r rhotacism – Alex B. Aug 23 '18 at 15:07
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    Note that the adjective is first/second, not third: foedus, -a, -um. – Draconis Aug 23 '18 at 15:44
  • But the stem in Classical Latin is foeder-, just like vetus, veter-, and similar to corpus, corpor-. Draconis' answer gives the historical background. – Colin Fine Aug 23 '18 at 22:08
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The noun foedus originally (by which I mean late Proto-Italic going into early Latin) looked something like *foidos-. Note that that's the stem, with no ending attached.

Proto-Indo-European ablaut patterns would have meant the stem was *foidos- in the nominative and vocative and *foides- in the other cases, but I'm not sure if this ablaut was still relevant in Proto-Italic or if it had been levelled out; this answer is written as if it were already levelled out, for simplicity. The result is the same either way.

The third-declension endings were then added onto this. (I'm only showing the nominative, genitive, and ablative, but you can extrapolate the rest.)

foidos-∅
foidos-es
foidos-e

Note that the nominative ending for neuter third-declension nouns is ∅ (that is, nothing at all). Then rhotacism applied, changing S between vowels to R.

foidos
foidores
foidore

Another process turned all short vowels in medial open syllables (i.e. vowels in the middle of a word and not before two consonants) into i. But Latin didn't like ir, so this sequence became er. Note that this happened no matter what the vowel was, so it doesn't matter if the PIE ablaut was still relevant or not.

foidos
foideres
foidere

Finally, short o in the last syllable of a word turned into u, short e in closed final syllables turned into i, and oi was respelled oe.

foedus
foederis
foedere

This led to the noun forms you're familiar with in Classical times.

The adjective foedus came from a different source: its stem was actually foid- instead of foidos-, and the standard first/second endings were tacked onto this.

  • foedus is a neuter -s stem, so there was never a second s for the nominative singular ("foidos-s"). – fdb Aug 23 '18 at 16:42
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    Can we be sure the oblique cases originally had *foidos- ? It looks parallel to genus/generis, which when compared to its Greek cognate γένος/γένεος suggests an original e/o ablaut. – varro Aug 23 '18 at 16:51
  • @fdb D'oh, I thought it was masculine for some reason. Lemme fix that. – Draconis Aug 23 '18 at 16:54
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    @luchonacho Which part, the -us/-eris pattern? It's quite common: latus/lateris "side", genus/generis "gender", sidus/sideris "star", just off the top of my head. – Draconis Aug 24 '18 at 15:05
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    @AlexB. I added a note about the PIE ablaut. Would you be interested in adding an answer to this question as well, which currently has nothing about ablaut in it? I'm curious if the ablaut has any effect on the difference between -eris S-stems like latus and -oris S-stems like corpus. TKR's analogical explanation seems convincing, but then again I don't know anything really about ablaut in noun declension. – Draconis Aug 28 '18 at 16:33

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