Spanish has many words containing the diphthongs /au/, /eu/ and /iu/, but the only instances of words containing /ou/ (as a diphthong or in hiatus) are a very small set of foreign loanwords:

bou, proustiano, soul, noúmeno

or compound words of the form ...o + u...:

estadounidense, estadounidismo, genitourinario, finoúgrio

The neighbouring romance languages Portuguese and Catalan both have a large number of words containing /ou/, is there a specific reason why Spanish doesn't?

There even seems to be a tendency to convert loan /ou/ to /o/, as in the Spanishised name of the Galician city Ourense (Orense), and its associated demonyms.

(My immediate thought was that all instances of Latin /ou/ became /oβ/ ~ /ob/ in Spanish, but this seems to be contradicted by the fact that /au/ and /eu/ remained in many words.)

  • 1
    to be sure, noúmeno is not a diphthong /ou/ but disyllabic /o.ˈu/ Commented May 8, 2018 at 0:55
  • 2
    Where should it come from? Latin did not have an ou diphthong (but au and eu), and Latin ob- is preserved in Spanish as /oβ/ ~ /ob/. Commented May 8, 2018 at 9:31
  • @jknappen Apologies, I didn't mean specifically diphthongs but all occurrences of /ou/, including as hiatus e.g. bovis > bou; novus > nou (Cat.). Looking at these examples I'm thinking that the case may be that many instances of Latin 'ou' diphthongised the 'o' as 'ue' (as it occurred on a stressed syllable e.g. buey, nuevo)
    – iacobo
    Commented May 8, 2018 at 10:07

2 Answers 2


It seems this was a combination of:

  1. 'ou' being rare in Latin words and only in environments where vowels would undergo changes in the evolution to Spanish, and
  2. instances of vowel + consonant transmuting into vowel + u/w in the evolution from Spanish to Latin being lost with back vowels (like o):

8. Adjustments due to vowel syncope


Finally, syllable-final /b/ (realized as [β]) was generally semivocalized to [w], although this latter sound was later lost through assimilation if it followed a back vowel (see the codo example below):

  • dēb(i)ta → [ˈdeβða] debda → [ˈdewða] deuda ‘debt’
  • cap(i)tālem → [kaβˈðal] cabdal → [kawˈðal] caudal ‘money, river flow’
  • cŭb(i)tum → [ˈkoβðo] cobdo → [ˈkowðo] → [ˈkoðo] codo ‘elbow’

Also, absentiaausencia, baptistabautista, rapĭdusraudo etc

So, the environments where /ow/ ~ /o.u/ existed in the past/exist in other romance cognates but don't occur in Spanish are:

  1. a) /o/ monothong stressed, /o/ diphthongized to /we/

    ovum → huevo
    bovis → buey
    novus → nuevo

    b) /o/ monothong unstressed, /w/ → /β/

    bovīnus → bovino

  2. a) /oβ/ → /ow/ diphthong, assimilates to /o/

    [ˈkðo] → [ˈkowðo] → [ˈkoðo]

    b) /ob/ preserved

    obcaecāre → obcecar


Two things:

1- The words you have listed are not Spanish, but loans, adaptations from ancient words or neologisms based on modern languages, foreign surnames, etc. 2- Ourense was never a loan into Spanish. As a matter of fact, the artificial revival of Galician forced a Galician pronunciation in places which had already lost it. Having lived in multilingual Spain for many years in my childhood (the real multilingual areas, not what the propaganda and official story tells you), I can tell you noone said Ourense or A coruña back then, at a time in which you could hear Girona, Xixón por other place names. Spanish never reduced Ou to O in Ourense. That has nothing to do with, say, the reanalysos of (O) Porto as Oporto.

2-Yes, there is a specific reason: it is called monophthongisation in the phonological diachrony of the language. As simple as that. Or you can see it as an issogloss in dialectological terms.

  • 1
    monodipthongation is not a term I know. Did you mean monophthongisation? Commented May 8, 2018 at 15:43
  • Yes, I did...sorry..sometimes languages mix in my head and I get confused....Your are right. Commented May 8, 2018 at 20:19
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    @JavierArias, comments are intended to improve the post. Once you know how to improve it, please edit the post, do not post yet another comment. Commented May 8, 2018 at 23:15

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