Are vowels long before "gn"?

Question

Allen and Greenough, §10d, provide a general rule:

A vowel before ns, nf, gn, is long: as in cōnstāns, īnferō, māgnus [emphasis modified]

This seems to agree with Priscian:

'gnus' quoque vel 'gna' vel 'gnum' terminantia longam habent vocalem paenultimam, ut 'rēgnum', 'stāgnum', 'benīgnus', 'malīgnus', 'abiēgnus', 'privīgnus', 'Pelīgnus'. (Keil, II.82)

However, Priscian does not mention magnus, and Lewis and Short disagree with A&G and say it has a short vowel. Interestingly, Bennet includes ns and nf in his parallel rule, but does not mention gn.

So which is it? Are vowels before gn generally long or not?

Answer 1

First of all, it's important to note that syllables containing a vowel + gn combination are long (or, less confusingly, "heavy"), regardless of the length of the vowel itself. As Bennet says:

A syllable is long,—
a) if it contains a long vowel; [...]
c) if it contains a short vowel followed by x, z, or any two consonants (5.B.1)

It does not follow, however, that the vowel preceding gn is necessarily long. For that, we'll need to turn to other sources.

W. Sydney Allen addresses the matter in Vox Latina, 71ff:

It is commonly, and mistakenly, believed that vowels in Latin are regularly long before the consonant-group gn.

Allen contends that:

• The "rule" rests on a single passage in Priscian
• The passage in Priscian is an interpolation, not original
• Historical evidence frequently contradicts the rule
• Only a few words—rēgnum, stāgnum, sēgnis, and abiēgnus—are likely to have a long vowel preceding gn

First, Allen addresses Priscian, and argues that the passage quoted above, in Keil II.82, is an interpolation first based on context:

The interpolatory nature of the 'gnus' passage is strongly suggested by its irrelevant interruption of the discussion of proper names, more particularly those with adjectives in -īnus; and by introduction of nouns into a chapter which is exclusively concerned with adjectives.

The interpolator, Allen continues, misinterpreted Priscian's discussion of Anagnīnus, where the vowel following gn, not preceding it, is long. Furthermore, the examples of the rule are not convincing: in several we would expect a long vowel for historical/phonological reasons, while many common words, for which we have evidence of a short vowel, are not mentioned. Allen summarizes:

It seems most probable, therefore, that what the interpolator states as a general rule is not so, and that he was at a loss to cite much more by way of example.

Turning to inscriptions, Allen says "we nowhere find even so common a word as magnus marked with a long a." There is some evidence for a long i in a few of the interpolator's examples, but Allen suggests that this could be explained by phonetic influences on the short i before [ŋ] that made its quality (not length) approach that of the long ī. This explanation is corroborated by long ī appearing occassionally in front of other inscriptional instances of [ŋ], like ng, nc and nqu.

Thus, Allen concludes:

We may safely say, then, that the vowel is long in rēgnum, stāgnum, sēgnis, abiēgnus, but probably NOT before gn in any other instances.

Answer 2

There is some inscriptional evidence that has been interpreted as indicating that short -ĭ- before -gn- was lengthened to -ī-, or perhaps remained short but was raised in quality from [ɪ] to [i].

That said, this evidence seems fairly weak and ambiguous to me. Furthermore, the testimony of the Romance languages indicates that any lengthening or raising of -ĭ- that occurred in this context was either subsequently reversed, or simply did not run to completion: for example, Italian has [e] in degno and segnare. Most recent sources that I've read only mark a vowel before -gn- with a macron if it is etymologically long. But for some recent contrary views, see Alex B.'s answer to What is the correct vowel quantity for words formed from sĭ̄gn-?

Relevant inscriptional evidence is mentioned in Allen 1989 (Vox Latina, pp. 72-73; cited in Nathaniel's answer) and Leumann 1977 (Lateinische Laut- und Formenlehre, §127, p. 113). I think the most complete description I've found so far is in Lindsay 1894 (The Latin Language: An Historical Account of Latin Sounds, Stems and Flexions, p 138), which mentions:

• prIvIgno (Corpus Inscriptionum Latinarum vi. 3541)
• dIgne (Corpus Inscriptionum Latinarum vi. 6314)
• sIgnum (C.I.L. vi. 10234), sIgnificabo (vi. 16664)

Leumann mentions inscriptions seignum "D 44" and "sIgnum 353": I'm not sure what the source for these is (the table of abbreviations says D = Diehl).

I think this evidence is arguable because even though the tall i symbol I, called i longa, is thought to normally represent long /iː/, it was not exclusively reserved for this function, and there is also some possibility of accidental use. Fortson 2008 says of i longa

one nonetheless has to be careful with interpreting this symbol: in many cases it was merely decorative, being used for I's that stood at the beginning of a paragraph or in certain high-profile words such as Imperator. See the discussion and examples in Christiansen 1889. I examined the difficulty of interpreting the use of both I longa and the apex (a symbol used to mark vowel length) in Fortson 2004, after the completion of which another study on this topic came to my attention, García González 2001, unfortunately unavailable to me and not published in the proceedings of the conference at which it was given.

(Language and Rhythm in Plautus: Synchronic and Diachronic Studies, Benjamin W. Fortson IV, page 127)

---

*A separate sound change that raised etymological *ĕ to i before -gn- was clearly complete already in Classical Latin. This is usually interpreted as a change of *ĕŋ to ĭŋ (found also in the context of original *ĕŋg and *ĕŋk). What's arguable is whether -ĭgn- then underwent a further change from [ɪŋn] to [iːŋn] or [iŋn].