I stumbled across this question on the pronunciation of 'vacuum' in the “linguistics” forum.

My question is: If uacuus is *wak+wo- why does uacuus have three syllables, but uiuus, paruus, caluus etc. have only two? Is there a rule that says that *wu remains after sonants (including l, r) but becomes *uu otherwise?

  • 2
    "Equus" might be an interesting one to compare against.
    – Draconis
    Commented Apr 8, 2017 at 14:47

2 Answers 2


The adjective uacuus is formed from the verb uacō (“I am empty, void”) with the help of the ‎adjective-forming suffix -uus. This suffix has 2 variants, the 1-syllable one and the 2-syllable one.

The 1-syllable form -vus is used after vowels (flā-vus, vī-vus), after L (cal-vus), and R (par-vus, cor-vus), and the 2-syllable form of that suffix, -uus is used after all other other consonants, like in your word vac-uus, with the exception of QU. In this last case, the form -us is used (multiloqu-us, obliqu-us).

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    Some more examples (from Weiss 297-8) supporting this pattern: salvus, clīvus, rāvus, rīvus, fulvus, furvus, helvus, arvus vs. caeduus, continuus, dīviduus, exiguus, irriguus, pāscuus, praecipuus.
    – TKR
    Commented Apr 8, 2017 at 16:38
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    @AlexB. I think Yellow Sky is using the "v" and "u" to differentiate between the one syllable and two syllable forms. The Romans would not have differentiated the symbols, but we, in the modern day, do so that is what he is getting at (1 syllable following vowels and liquids, 2 syllables after consonants).
    – Sam K
    Commented Apr 9, 2017 at 1:44

Yes, this is a well-known rule (see Leumann 1977: 132-133, Tronskii 1960: 101, Weiss 2009/2011: 124 and many others).

Weiss puts it this way,

“An unstressed short vowel between a liquid and u̯ is syncopated” (p. 124):

*bholh1i-u̯os > *foliu̯os > fulvus

*sl̩h₂u̯os (or *sl̩h₂euos) > *salau̯os > salvus

*arau̯om > arvum

cf. Leumann "Postkons. lat. u̯ (v) vor Vokal aus vokal. u: vorhistorisch hinter r l; hinter anderen Konsonanten aus Versnot bei Daktylikern, spaeter allgemein im Vulgaerlatein" (p. 132).

It occurred in the pre-literary period.


“If the vowel in the preceding syllable was long, the syncope did not occur until the post-Plautine period:

mīluus > mīlvus

lārua > lārva

*pēleu̯is > pēluis > pēlvis

  • I don’t see any evidence for *sala-wus, with or without syncopation. The cognates (e.g. Skt sarva-, Gk. *holwos > holos) point to *salw-us.
    – fdb
    Commented Apr 9, 2017 at 17:57
  • @fdb in Latin -lu̯- develops to -ll- (hence, sollus). But salvus is usually explained with syncope (thus,-lu̯- remains). See Leumann para 60 or Weiss (p. 162, point 9). NB: Unable to fix formatting/italics/reconstructed forms.
    – Alex B.
    Commented Apr 9, 2017 at 18:19
  • @fdb I assume you've looked it up in Beekes or de Vaan? Because they do address the point you raised. (Beekes: *slh2-eu-)
    – Alex B.
    Commented Apr 9, 2017 at 22:22
  • de Vaan actually writes *slH-u- with unidentified laryngeal. The laryngeal is posited to explain the /a/ in the first syllable.
    – fdb
    Commented Apr 9, 2017 at 22:32
  • 1
    Ah, that's the sound change I was missing! That makes sense, thanks.
    – TKR
    Commented Apr 13, 2017 at 21:15

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