The laryngeals were three consonant sounds that appear in most current reconstructions of the Proto-Indo-European language. The theory was first proposed by Ferdinand de Saussure in 1879; however, it did not begin to achieve any general acceptance until Hittite was discovered and slowly deciphered in the mid-20th century. It soon became apparent that Hittite had phonemes for which the laryngeal theory was the best explanation, and as such the laryngeal theory is accepted by most Indo-Europeanists.
The existence of these sounds was not suspected for quite some time, because Hittite and the Anatolian languages are the only Indo-European languages in which they ever survive as actual phonemes that appear in the records we have of those extinct languages. That said, the existence of these sounds is now accepted by most philologists, because positing their existence simplifies some otherwise hard to explain sound changes that appear in the descendant languages of PIE.
There were three such laryngeals:
In Greek, between consonants h1 > e, h2 > a, and h3 > o. In Indo-Iranian languages such as Sanskrit, each laryngeal becomes i, and in all other Indo-European languages, each laryngeal becomes a. This explains such observed phenomena as:
The chief evidence of laryngeals was the fact that when they appeared in connection with the PIE vowel *e-, h2 coloured it to *a-, and h3 to *o-. In Anatolian, however, h2 was preserved, and h3 was preserved in certain positions. For example:
The laryngeal theory has been posited as the best explanation of the otherwise mysterious appearance of h- in the Anatolian words, and the vowel difference between the Anatolian languages and most other Indo-European languages, such as Latin ovis.
Further evidence of the laryngeals comes from Uralic (Finno-Ugric) languages. While Proto-Uralic was typologically and grammatically very different from PIE and thus genetically unrelated, some words reconstructed into Uralic 'proto-dialects' (such as Proto-Finno-Ugric, Proto-Finno-Permic etc.) have been borrowed from PIE-dialects (cf. Finnish nimi <=> name and porsas<=> pork). After assuming that PIE laryngeals could have translated into guttural phonemes in the borrowing language, new loan words are being revealed in increasing numbers.
Three Uralic phonemes turn up in positions where PIE had laryngeals. Unfortunately Uralic, which was rich in alveolars, had few guttural phonemes to chose from. In post-vocalic positions both the post-alveolar fricatives that ever existed in Uralic are represented, an extinct (velar?) one in the very oldest borrowings and a grooved one (*/sh/ as in shoe becoming modern Finnic /h/) in some younger ones. The velar plosive /k/ is the third correspondance and the only one found word-initially, as is to be expected under relevant Uralic phonological limitations. Thus Finnish teh- / teke- (to do) is a borrowing from PIE *dheh1- > Proto-Germanic *do:n (to do), Finnish lehti (leaf, sheet) <= PIE *bhlh1-to (Uralic never reflects but the last consonant in initial clusters) giving later scandinavian 'blad' (blade, leaf, sheet) as well as Finnish kal-ja (week beer) derived by suffix from *kale- <= PIE *h2alu- giving engl. ale and scand. öl (beer).
The laryngeal theory requires fairly widespread adjustments in our view of the inflections of Indo-European. We now know that PIE may have had two original grammatical genders, masculine and neuter. The feminine gender that most of the oldest Indo-European languages share may have been formed by a suffix, *-eh2, which was coloured by the laryngeal to *-a. This makes the feminine nouns and adjectives originally consonant stems rather than original vowel stems, and helps explain why they are inflected differently from other nouns that are true vowel stems.
The laryngeal theory also explains a number of different ablaut sequences that appear in many Indo-European roots, and makes them seem less arbitrary and more regular. For example, the observed sequences:
Considerable debate still surrounds the pronunciation of the laryngeals. Many believe that they represented some sort of glottal stop or pharyngeal; others assume that *h2 was a velar and *h3 was a labiovelar fricative. Unfortunately, the reflexes in Uralic languages would be the same whatever of the former we prefer. The Uralic (and Hittite) evidence does however seem to contradict the assumption still held by some, that the laryngeals would have had no pronunciation at all, and are simply phonetic coefficients.
Anhtoer theory is that in Arabic the laryngeals are still alive and well at the present day:-