I have studied and known Spanish my whole life, and got a job at a University where I am allowed to take some free classes. Over the past three years, I have taken all the Italian classes offered, all the Arabic classes, and now I am currently enrolled in a Portuguese class.

One thing that I am beginning to find really odd is how Spanish makes extensive use of diphthongs whereas these other two Romance languages do not. Is there a historical reason for this?

Some examples off the top of my head are:

ES: No puedo ir mañana.

ES: Yo juego al béisbol.

PT: Não posso ir amanhã.

PT: Eu jogo beisebol.

IT: Non posso andare domani.

IT: Io gioco a baseball.

With الديوان, Spanish uses a diphthong, but the diphthong [ua] is already almost there, just preceded by an [i].

ES: aduana

AR: ad-diwan

But in الطوب, there is just a change in the long [u] to a more informal [o]

ES: adobe

AR: aṭ-ṭuub

Overall from Arabic, there doesn't appear to be any discernible reason from a Romance Language point of view as to what happens to the vowels, although if you can understand Egyptian Arabic, it looks pretty normal.

  • Note: the 'ua' in 'aduana' is not a diphthong but a hiatus. The word has four syllables: 'a·du·á·na'. This is quite irregular since 'ua' is usually a diphthong, as in 'cuá·dro'; perhaps a remnant from its arab origin. Commented Aug 14, 2017 at 14:31

3 Answers 3


This is a sound change operating on stressed vowels, variations of which arose in the daughter languages of Latin. The fact that the sound change only operates on stressed vowels explains occasional alternations (e.g. juego vs jugamos).

Similar vowel-breaking phenomena arose in Spanish and French, but not in Portuguese or Catalan. Hence, Latin petra > Italian pietra, French pierre, Portuguese pedra. Also, vowel-breaking does indeed happen in Italian: consider Latin focus > It. fuoco, or L. homo > It. uomo. Although it doesn't appear as often in the verbal paradigm as it does in Spanish, there is the alternation posso vs può, and similarly voglio vs vuole.

As for the question of why the variability exists in the daughter languages of Latin, the paper From hiatus to diphthong: the evolution of vowel sequences in Romance by Ioana Chitoran and José Ignacio Huald, attempts to tie it to the phonetic inventory of each language.

As for your last point, I'm not sure there is a relationship between this sound change and the nature of Arabic borrowings. Certainly Arabic borrowings such as Sp. adobe have a monophthong in stressed position.

  • Thanks, this is a good paper. E.g. I had no idea that some Romanian palatalization arose from earlier vowel breaking. Cool. Commented Mar 7, 2012 at 2:41

Interestingly, Portuguese has just about the same number of phonemic diphthongs as Spanish, but in Portuguese they are all falling diphthongs (these are EP):

[aj] [ɐj] [ɔj] [oj] [uj] [ɐ̃j] [õj] [ũj] (falling)
[aw] [ow] [ew] [ɛw] [iw] [ɐ̃w] (falling)

While in Spanish we find both kinds: `

[aj] [ej] [oj] (falling)
[aw] [ew] [ow] (falling)
[ja] [je] [jo] [ju] (rising)
[wa] [we] [wi] [wo] (rising)

But perhaps diphthongs are more 'noticeable' in Spanish because of the active sound change that jogloran describes in his answer (vowel breaking). In Spanish there are verb conjugations like:

sEntir <-> sIEnto <-> sIntiendo
pOder <-> pUEdo <-> pUdiendo

where the diphthongs only appear in certain forms. Portuguese does not do this, so it may seem that Portuguese uses diphthongs less.

jogloran points out how this arose historically in Spanish and French and to a lesser degree in Italian, but not in Portuguese and Romanian. The Portuguese diphthongs arose from an entirely different mechanism.

In Spanish, the words borrowed from Arabic came into Spanish after the vowel breaking from Vulgar Latin had already happened, and was no longer active. So any diphthongs in Arabic borrowings came from sounds already present in the Arabic.

  • 1
    This may simply be because of the different approach taken by Portuguese in the analysis. The io in the man’s name Mário (PT) are considered to be in hiatus; hence the marked accent on the first syllable. In the identically pronounced Mario (ES), they are considered a rising diphthong, and thus a word of two syllables that needs no written accent mark.
    – tchrist
    Commented Aug 4, 2012 at 20:49
  • 1
    Brazilian Portuguese also has the diphthong [ej], as in leite ['lejt͡ʃi], ("milk"). By the way, I think that Portuguese does have rising diphthongs. For example, [wa] in the word quase ['qwazi] ("almost"). Or am I missing something here? Commented Feb 6, 2013 at 22:49

(Brazilian) Portuguese does have [we], (in words that were spelled in old spelling with -güe-, -qüe, -güi, -qüi- ): agüentar, agüar, líqüido, sangüíneo, birigüi etc...; and in toponyms of Amerindian and African origin : Ibirapuera, Birigui (former spelling: Birigüi), Itapuã...

Brazilian Portuguese has so many diphthongs which are not indicated in spelling: vocês [vo'sejs], Brasil [bra'ziw], sulco (['suwku] different than suco ['suku])...Ruim ''bad'' is frequently pronounced as a monosyllabic word, so it sounds like a nasalized version of the name Ruy [huj] (with both u and j nasalized).

Different than French (or maybe even Continental Portuguese), voar ''to fly'' is [vu'a(h)] or [vo'a(h)] Monosyllabic [vwah] (à la French voix [vwax]) is not common in Brazilian Portuguese (but it's frequent in Continental Portuguese [vwar]).

Going back to Spanish, in standard peninsular Spanish (Madrid/Salamanca) muy is [mwi] and not [muy]: http://www.wordreference.com/espt/muy

  • 1
    Your final point is covered by the rule that the weak vowels i and u (or the other way round) combine to form a falling diphthong as in cuidado [kwiˈð̞a.ð̞o] and ciudad [θjuˈð̞að̞]
    – Henry
    Commented Jul 4, 2023 at 21:17

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