I found an Indo-European root, nō̆-men-, in the free dictionary.
I know both macron and breve mean the length of vowels. And the Latin word, nōmen, has a long ō.
However, how should I pronounce the vowel with macron and breve simultaneously?
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This root appears in Latin (and other languages) with a long ō, nōmen, but in Greek (and others) with a short o, ónoma. So it's not entirely clear whether the PIE vowel was long or short. We know it was one or the other, but there's not a consensus on which one.
One theory is that it was short in PIE, and became long separately in Indo-Iranian (via Brugmann's Law), Germanic (via Germanic ablaut), and Latin (by analogy with other words: for example, cōgnōmen looks like cognōsco, which is from a separate root with a long ō). But Brugmann's Law is far from universally accepted, and others argue that the root had a long ō which became short in some languages.
The linked entry in “the free dictionary” has the lemma as “nō̆-men-“, but then proceeds to claim “oldest form *h1no(h3)-mn̥”. If *h1no(h3)-mn̥ is Proto-Indo-European (PIE) what language is nō̆-men- supposed to be? Surely not Latin? If not, what then is "oldest form" supposed to mean?
The “Leiden school”, as exemplified by de Vaan’s etymological dictionary, posits the PIE form *h3neh3-mn-, zero-grade *h3nh3-mn-. The first laryngeal would explain the initial o- in Greek onoma; the second laryngeal would account for the long vowel in Latin nōmen. This theory relieves us of having to posit an ambiguous vowel in the first syllable.