missing (examples including Dutch, Ukrainian, Arabic, Thai, and Vietnamese); on the other hand, if a labial stop is missing, the voiceless is the most likely candidate (examples including stages in the evolution of Irish and Japanese, and Arabic). Secondly, there are few languages which have breathy voiced consonants but no voiceless aspirates and even fewer that simultaneously contrast breathy voice with full voice. Roman Jakobson has asserted that no such language is known; however, that is disputed by some linguists who oppose the theory. For example, Robert Blust showed that a system of voiceless, voiced and voiced aspirated (not murmured) stops, as postulated in the traditional reconstruction of Proto-Indo-European, exists in Kelabit, a language of the Sarawak highlands in Borneo. Others have observed, however, that the sounds in Kelabit are not actually murmured but voiceless with breathy release and so are not comparable to what is posited for Proto-Indo-European. In any event, the traditional reconstruction remains a typological oddity. Thirdly, a longstanding but unexplained observation of Indo-Europeanists about the distribution of stops in word roots is that it had long been noted that certain combinations of consonants were not represented in Proto-Indo-European words in terms of the traditional system: No root contained a sequence of two plain voiced stops: there were no roots of the type , which reflects a common cross-linguistic constraint against roots with two similar stops. No root contained both a voiceless stop and a voiced aspirate: roots of the type or were not attested. As the result of a sound shift from Pre-Proto-Indo-European, roots of this type are attested as and instead. On the other hand, the plain voiced stops were compatible with either of the other two series: or were both possible. The constraints on the phonological structure of the root cannot be explained in terms of a theory of unlimited assimilation or dissimilation since they display a radical difference in patterning between three sets of consonants, the stops, that ought to behave identically. Typologically, that is very odd. Original glottalic proposal The glottalic theory proposes different phonetic values for the stop inventory of Proto-Indo-European: In his version of the glottalic theory, Hopper (1973) also proposed that the aspiration that had been assumed for the voiced stops could be accounted for by a low-level phonetic feature known to phoneticians as "breathy voice". The proposal made it possible both to establish a system in which there