The evidence for them is mostly indirect, but serves as an explanation for differences between vowel sounds across Indo-European languages. For example, Sanskrit and Ancient Greek, two descendents of PIE, exhibit many similar words that have differing vowel sounds. Assume that the Greek word contains the vowel [e] and the corresponding Sanskrit word contains [i] instead. The laryngeal theory postulates these words originally had the same vowels, but a neighboring consonant which had since disappeared had altered the vowels. If one labeled the hypothesized consonant as [h1], then the original PIE word may have contained something like [eh1] or [ih1], or perhaps a completely different sound such as [ah1].
The original phonetic values of the laryngeal sounds remain controversial (See below).
The late discovery of these sounds by Indo-Europeanists is largely due to the fact that Hittite and the other Anatolian languages are the only Indo-European languages where at least some of them are attested directly and consistently as consonantal sounds. Otherwise, their presence is to be seen mostly through the effects they have on neighboring sounds, and on patterns of alternation that they participate in; when a laryngeal is attested directly, it is usually as a vowel (as in the Greek examples below). Most Indo-Europeanists accept at least some version of laryngeal theory because their existence simplifies some otherwise hard-to-explain sound changes and patterns of alternation that appear in the Indo-European languages, and solves some minor mysteries, such as why verb roots containing only a consonant and a vowel have only long vowels e.g. *dō- "give"; re-reconstructing *deh₃- instead not only accounts for the patterns of alternation more economically than before, but brings the root into line with the basic consonant - vowel - consonant Indo-European type.
There are many variations of the Laryngeal theory. Some scholars, such as Oswald Szemerényi, reconstruct just one. Some follow Jaan Puhvel's reconstruction of eight or more (in his contribution to Evidence for Laryngeals, ed. Werner Winter). Most scholars work with a basic three:
Many scholars, however, either insist on or allow for a fourth consonant, *h₄, which differs from *h₂ only in not being reflected as Anatolian ḫ. Accordingly, except when discussing Hittite evidence, the theoretical existence of an *h₄ contributes little. Another such theory, but much less generally accepted, is Winfred P. Lehmann's view, on the basis of inconsistent reflexes in Hittite, that *h₁ was actually two separate sounds. (He assumed that one was a glottal stop and the other a glottal fricative.)
Some direct evidence for laryngeal consonants from Anatolian:
PIE *a is a rarish sound, and in an uncommonly large number of good etymologies it is word-initial. Thus PIE (traditional) *anti "in front of and facing" > Greek antí "against"; Latin ante "in front of, before"; (Sanskrit ánti "near; in the presence of"). But in Hittite there is a noun ḫants "front, face", with various derivatives (ḫantezzi "first", and so on), pointing to a PIE root-noun *h₂ent- "face" (of which *h₂enti would be the locative singular). (It does not necessarily follow that all reconstructed forms with initial *a should automatically be rewritten *h₂e.)
Similarly, the traditional PIE reconstruction for 'sheep' is *owi- (a y-stem, not an i-stem) whence Sanskrit अवि-, Latin ovis, Greek ὄϊς. But now Luwian has 𒄩𒀀𒌑𒄿𒅖-, indicating instead a reconstruction *h₃ewi-.
But if laryngeals as consonants were first spotted in Hittite only in 1935, what was the basis for Saussure's conjectures some 55 years earlier?
They sprang from a reanalysis of how the patterns of vowel alternation in Proto-Indo-European roots of different structure aligned with one another.
A feature of Proto-Indo-European morpheme structure was a system of vowel alternations christened ablaut ("alternate sound") by early German scholars and still generally known by that term (except in French, where the term apophonie is preferred). Several different such patterns have been discerned, but the commonest one, by a wide margin, is e/o/zero alternation found in a majority of roots, in many verb and noun stems, and even in some affixes (the genitive singular ending, for example, is attested as *-es, *-os, and *-s). The different states are called ablaut grades; e-grade and o-grade are together "full grades", and the total absence of any vowel is "zero grade".
Thus the root *sed- "to sit (down)" (roots are traditionally cited in the e-grade, if they have one) has three different shapes: *sed-, *sod-, and *sd-. This kind of patterning is found throughout the PIE root inventory and is transparent:
Now, in addition to the commonplace roots of consonant + vowel + consonant structure there are also well-attested roots like *dhē- "put, place": these end in a vowel, which is always long in the categories where roots like *sed- have full grades; and in those forms where zero grade would be expected, before an affix beginning with a consonant, we find a short vowel, reconstructed as *ə, or schwa (more formally, schwa primum indogermanicum). The cross-language correspondences of this vowel are different from the other five short vowels. (Before an affix beginning with a vowel, there is no trace of a vowel in the root, as shown below.)
Whatever caused a short vowel to disappear entirely in roots like *sed-/*sod-/*sd-, it was a reasonable inference that a long vowel under the same conditions would not quite disappear, but would leave a sort of residue. This residue is reflected as i in Indic while dropping in Iranian; it gives variously e, a, o in Greek; it mostly falls together with the reflexes of PIE *a in the other languages (always bearing in mind that short vowels in non-initial syllables undergo various adventures in Italic, Celtic, and Germanic):
Conventional wisdom lined up roots of the *sed- and *dō- types as follows:
|Full Grades||Weak Grades|
But there are other patterns of "normal" roots, such as those ending with one of the six resonants (*y w r l m n), a class of sounds whose peculiarity in Proto-Indo-Eruopean is that they are both syllabic (vowels, in effect) and consonants, depending on what sounds are adjacent:
Root *bher-/bhor-/bhṛ- ~ bhr- "carry"
Saussure's insight was to align the long-vowel roots like *dō-, *stā- with roots like *bher-, rather than with roots of the *sed- sort. That is, treating "schwa" not as a residue of a long vowel but, like the *r of *bher-/*bhor-/*bhṛ-, an element that was present in the root in all grades, but which in full grade forms coalesced with an ordinary e/o root vowel to make a long vowel, with "coloring" (changed phonetics) of the e-grade into the bargain; the mystery element was seen by itself only in zero grade forms:
|Full Grades||Zero Grade|
|bher-, bhor-||bhṛ- / bhr-||"carry"|
|deX, doX-||dẊ- / dX-||"give"|
Saussure treated only two of these elements, corresponding to our *h₂ and *h₃. Later it was noticed that the explanatory power of the theory, as well as its elegance, were enhanced if a third element were added, our *h₁, which has the same lengthening and syllabifying properties as the other two but has no effect on the color of adjacent vowels. Saussure offered no suggestion as to the phonetics of these elements; his term for them, "coéfficients sonantiques", was not however a fudge, but merely the term in general use for glides, nasals, and liquids (i.e., the PIE resonants) as in roots like *bher-.
As mentioned above, in forms like *dwi-bhr-o- (etymon of Greek diphrós, above), the new "coéfficients sonantiques" (unlike the six resonants) have no reflexes at all in any daughter language. Thus the compound *mṇs-dheH- "to 'fix thought', be devout, become rapt" forms a noun *mṇs-dhH-o- seen in Proto-Indo-Iranian *mazdha- whence Sanskrit medhá- /mēdha/ "sacrificial rite, holiness" (regular development as in sedur < *sazdur, above), Avestan mazda- "name (originally an epithet) of the greatest deity".
There is another kind of unproblematic root, in which obstruents flank a resonant. In the zero grade, unlike the case with roots of the *bher- type, the resonant is therefore always syllabic (being always between two consonants). An example would be *bhendh- "tie, bind":
This is all straightforward and such roots fit directly into the overall patterns. Less so are certain roots that seem sometimes to go like the *bher- type, and sometimes to be unlike anything else, with (for example) long syllabics in the zero grades while at times pointing to a two-vowel root structure. These roots are variously called "heavy bases", "dis(s)yllabic roots", and "seṭ roots" (the last being a term from Pāṇini's grammar. It will be explained below).
For example, the root "be born, arise" is given in the usual etymological dictionaries as follows:
The (A) forms occur when the root is followed by an affix beginning with a vowel; the (B) forms when the affix begins with a consonant. As mentioned, the full-grade (A) forms look just like the *bher- type, but the zero grades always and only have reflexes of syllabic resonants, just like the *bhendh- type; and unlike any other type, there is a second root vowel (always and only *ə) following the second consonant:
On the term "seṭ". The Pāṇinian term "seṭ" (that is, sa-i-ṭ) is literally "with an /i/". This refers to the fact that roots so designated, like jan- "be born", have an /i/ between the root and the suffix, as we've seen in Sanskrit jánitar-, jániman-, janitva (a gerund). Cf. such formations built to "aniṭ" ("without an /i/") roots, such as han- "slay": hántar- "slayer", hanman- "a slaying", hantva (gerund). In Pāṇini's analysis, this /i/ is a linking vowel, not properly a part of either the root or the suffix. It is simply that some roots are in effect in the list consisting of the roots that (as we would put it) "take an -i-".
But historians have the advantage here: the peculiarities of alternation, the "presence of /i/", and the fact that the only vowel allowed in second place in a root happens to be *ə, are all neatly explained once *ǵenə- and the like were understood to be properly *ǵenH-. That is, the patterns of alternation, from the point of view of Indo-European, were simply those of *bhendh-, with the additional detail that *H, unlike obstruents (stops and *s) would become a syllable between two consonants, hence the *ǵenə- shape in the Type (B) formations, above.
The startling reflexes of these roots in zero grade before a consonant (in this case, Sanskrti ā, Greek nē, Latin nā, Lithuanian ìn) is explained by the lengthening of the (originally perfectly ordinary) syllabic resonant before the lost laryngeal, while the same laryngeal protects the syllabic status of the preceding resonant even before an affix beginning with a vowel: the archaic Vedic form jajanur cited above is structurally quite the same (*ǵe-ǵṇh₁-ṛ) as a form like *da-dṛś-ur "they saw" < *de-dṛḱ-ṛ.
Incidentally, redesigning the root as *ǵenH- has another consequence. Several of the Sanskrit forms cited above come from what look like o-grade root vowels in open syllables, but fail to lengthen to -ā- per Brugmann's law. All becomes clear when it is understood that in such forms as *ǵonH- before a vowel, the *o is not in fact in an open syllable. And in turn that means that a form like jajāna "was born", which apparently does show the action of Brugmann's law, is actually a false witness: in the Sanskrit perfect tense, the whole class of seṭ roots, en masse, acquired the shape of the aniṭ 3sing. forms. (See Brugmann's law for further discussion.)
There are also roots ending in a stop followed by a laryngeal, as *pleth₂-/*pḷth₂- "spread, flatten", from which Sanskrit pṛthú- "broad" masc. (= Avestan pərəθu-), pṛthivī- fem., Greek platús (zero grade); Skt. prathimán- "wideness" (full grade), Greek platamṓn "flat stone". The laryngeal explains (a) the change of *t to *th in Proto-Indo-Iranian, (b) the correspondence between Greek -a-, Sanskrit -i- and no vowel in Avestan (Avestan pərəθwī "broad" fem. in two syllables vs Sanskrit pṛthivī- in three).
Stray laryngeals can be found in isolated or seemingly isolated forms; here the three-way Greek reflexes of syllabic *h₁, *h₂, *h₃ are particularly helpful, as seen below. (Comments on the forms follow.)
The Greek forms ánemos and árotron are particularly valuable because the verb roots in question are extinct in Greek as verbs. This means that there is no possibility of some sort of analogical interference, as for example happened in the case of Latin arātrum "plow", whose shape has been distorted by the verb arāre "to plow" (the exact cognate to the Greek form would have been *aretrum). It used to be standard to explain the root vowels of Greek thetós, statós, dotós "put, stood, given" as analogical. Most scholars nowadays probably take them as original, but in the case of "wind" and "plow", the argument can't even come up.
Regarding Greek híeros, the pseudo-participle affix *-ro- is added directly to the verb root, so *isḥ₁-ro- > *isero- > *ihero- > híeros (with regular throwback of the aspiration to the beginning of the word), and Sanskrit iṣirá-. There seems to be no question of the existence of a root *eysH- "vigorously move/cause to move". If the thing began with a laryngeal, and most scholars would agree that it did, it would have to be *h₁-, specifically; and that's a problem. A root of the shape *h₁eysh₁- is not possible. Indo-European had no roots of the type *mem-, *tet-, *dhredh-, i.e., with two copies of the same consonant. But Greek attests an earlier (and rather more widely-attested) form of the same meaning, híaros. If we reconstruct *h₁eysh₂-, all of our problems are solved in one stroke. The explanation for the híeros/híaros business has long been discussed, without much result; laryngeal theory now provides the opportunity for an explanation which did not exist before, namely metathesis of the two laryngeals. It's still only a guess, but it's a much simpler and more elegant guess than the guesses available before.
The syllabic *ḥ₂ in *pḥ₂ter- "father" might not really be isolated. Certain evidence shows that the kinship affix seen in "mother, father" etc. might actually have been *-h₂ter- instead of *-ter-. The laryngeal syllabified after a consonant (thus Greek patḗr, Latin pater, Sanskrit pitár-; Greek thugátēr, Sanskrit duhitár- "daughter") but lengthened a preceding vowel (thus say Latin māter "mother", frāter "brother") — even when the "vowel" in question was a syllabic resonant, as in Sanskrit yātaras "husbands' wives" < *yṆt- < *yṇ-h₂ter-).
Work particularly associated with research of the scholar Jorma Koivulehto has identified a number of additions to the list of Finnic loanwords from an Indo-European source or sources whose particular interest is the apparent correlation of PIE laryngeals with three post-alveolar phonemes (or their later reflexes) in the Finnic forms. If so, this would point to a great antiquity for the borrowings, since no attested Indo-European language neighbouring Uralic has consonants as reflexes of laryngeals. And it would bolster the idea that laryngeals were distinctly consonantal, phonetically.
Three Uralic phonemes reflect PIE laryngeals. In post-vocalic positions both the post-alveolar fricatives that ever existed in Uralic are represented: firstly a possibly velar one, theoretically reconstructed much as the PIE laryngeals (conventionally marked *x), in the very oldest borrowings and secondly a grooved one (*š as in shoe becoming modern Finnic h) in some younger ones. The velar plosive k is the third reflex and the only one found word-initially (a possible older word-initial reflex *x would have disappeared in Uralic without a trace). In intervocalic position the reflex k is probably younger than either of the two former ones. The fact that Finnic may have plosive reflexes for PIE laryngeals is to be expected under well documented Finnic phonological behaviour and does not mean much for tracing the phonetic value of PIE laryngeals (cf. Finnish Kansa ‘people’<= Gmc *χansā ’company, troupe, party, crowd’ (cf. German Hansa), Finnish kärsiä ‘suffer, endure’<= Gmc *χarđia- ’endure’ (cf. E. hard), Finnish pyrkiä <= PGmc. *wurk(i)ja- ‘work, work for’ etc).
The correspondences work in a perfectly equal way for h1, h2 and h3. Thus
1) PIE laryngeals correspond to PU fricative *x in wordstems like:
-Finnish nai-/naa- 'woman' < PU *näχi-/*naχi- <= PIE *[gwnah2-] = */gwneh2-/ > Sanskrit gnā́ 'goddess', OIr. mná (gen. of ben), ~Greek gunē ‘woman’(cognate to Engl. queen)
-Finnish tuo- 'bring' ~ Samoyed tāś 'give' < PU *toχi <= PIE *[doh3-] = */deh3-/ > Greek didōmi, Lat. dō-, Old Lith. dúomi 'give', Hittite dā 'take'
2) PIE laryngeals correspond to Pre-Finnic fricative *š in wordstems like:
-Finnish rohto 'medical plant, green herb' < PreFi *rošto <= PreG *groH-tu- > Gmc. *grōþu 'green growth' > Swedish grodd 'germ (shoot)'
-Old Finnish inhi-(m-inen) 'human being' < PreFi *inše- 'descendant' <= PIE *ģnh1-(i)e/o- > Sanskrit jā́- 'born, offspring, descendant', Gmc. *kunja- 'generation, lineage, kin'
3) PIE laryngeals correspond to Pre-Finnic *k in wordstems like:
-Finnish kesä- 'summer' < PFU *kesä- <= PIE *h1es-en- (*h1os-en-/-er-) > Balto-Slavic *eseni- 'autumn', Gothic asans 'summer'
-Finnish kaski 'burnt-over clearing' < PreFi *kaske / *kaśke <= PIE/PreG *[h2a(h1)zg-] = */h2e(h1)sg-/ > Gmc. *askōn 'ashes'
-Finnish koke- 'to perceive, sense' < PreFi *koke- <= PIE *[h3okw-ie/o] = */h3ekw-ie/o/ > Greek opsomai ‘look, observe’ (cognate to Lat. oculus ‘eye’)
-Finnish kulke- 'to go, walk, wander' ~ Hungarian halad- 'to go, walk, proceed' < PFU *kulke- <= PIE *kwelH-e/o- > Greek pelomai ’(originally) to be moving', Sanskrit cárati 'goes, walks, wanders (about)’, cognate Lat. colere 'to till, cultivate, inhabit'
-Finnish teke- 'do, make' ~ Hungarian tëv-, të-, tesz- 'to do, make, put, place' < PFU *teke- <= PIE *dheh1> Greek títhēmi, Sanskrit dádhāti 'put, place', but 'do, make' in the western IE languages, e.g. the Germanic forms do, German tun, etc., and Latin faciō (though OE dón and into Early Modern English still sometimes means "put", and still does in colloquial German).
These examples represent but a few examples of the lexical borrowings found, especially when one also considers a number of etymologies with laryngeal reflexes in Finno-Ugric languages other than Finnish. For most cases no other plausible etymology exists. While some single etymologies may be challenged, the case for this oldest stratum itself seems conclusive from the Uralic point of view, and corresponds well with all that is known about the dating of the other most ancient borrowings and about contacts with Indo-European populations. Yet acceptance for this evidence is far from unanimous among Indo-European linguists, some even regard the hypothesis controversial.
The following is a rundown of laryngeals in Proto-Indo-European morphology.
The same is shown by some IE-Semitic correspondences, whether these are due to prehistoric borrowing or to a common ancestor (see Nostratic theory):-