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In linguistics, a consonant cluster, consonant sequence or consonant compound is a group of consonants which have no intervening vowel. In English, for example, the groups /spl/ and /ts/ are consonant clusters in the word splits.
Some linguists[who?] argue that the term can be properly applied only to those consonant clusters that occur within one syllable. Others claim that the concept is more useful when it includes consonant sequences across syllable boundaries. According to the former definition, the longest consonant clusters in the word extra would be /ks/ and /tr/, whereas the latter allows /kstr/.
Languages' phonotactics differ as to what consonant clusters they permit.
Many languages are more restrictive than English in terms of consonant clusters. Many languages forbid consonant clusters entirely. Hawaiian, like most Malayo-Polynesian languages, is of this sort. Japanese is almost as strict, but allows a sequence of a nasal or approximant, plus another consonant, as in Honshū [hoɴɕuː] (the name of the largest island of Japan), and Tokyo [toːkjoː]. Standard Arabic forbids initial consonant clusters and more than two consecutive consonants in other positions. So do most other Semitic languages, although Modern Israeli Hebrew permits initial two-consonant clusters (e.g. pkak "cap"; dlaat "pumpkin"), and Moroccan Arabic, under Berber influence, allows strings of several consonants. Like most Mon–Khmer languages, Khmer permits only initial consonant clusters with up to three consonants in a row per syllable. Finnish has initial consonant clusters natively only on South-Western dialects and on foreign loans, and only clusters of three inside the word are allowed. Most spoken languages and dialects, however, are more permissive. In Burmese, consonant clusters of only up to three consonants (the initial and two medials—two written forms of /-j-/, /-w-/) at the initial onset are allowed in writing and only two (the initial and one medial) are pronounced. These clusters are restricted to certain letters. Some Burmese dialects allow for clusters of up to four consonants (with the addition of the /-l-/ medial, which can combine with the above-mentioned medials.
At the other end of the scale, the Kartvelian languages of Georgia are drastically more permissive of consonant clustering. Clusters in Georgian of four, five or six consonants are not unusual—for instance, /brtʼqʼɛli/ (flat), /mt͡sʼvrtnɛli/ (trainer) and /prt͡skvna/ (peeling)—and if grammatical affixes are used, it allows an eight-consonant cluster: /ɡvbrdɣvnis/ (he's plucking us). Consonants cannot appear as syllable nuclei in Georgian, so this syllable is analysed as CCCCCCCCVC. The neighboring, but unrelated, Armenian language also allows for long consonant strings. For example, the classic transliterations k̕rt̕mndzhal and khghchmtank̕ of քրթմնջալ /kʰɾtʰmnd͡ʒɑl/ ("to grumble") and խղճմտանք /χʁt͡ʃmtɑnkʰ/ ("conscience") start with eight consonants, though in the Armenian alphabet and the modern ISO 9985 transliteration words starting with more than six consonants are rare. Many Slavic languages may manifest almost as formidable numbers of consecutive consonants, such as in the Slovak words štvrť /ʃtvr̩tʲ/ ("quarter"), and žblnknutie /ʒbl̩ŋknutje/ ("clunk"; "flop") and the Slovene word skrbstvo /skrbstʋo/ ("welfare"). However, the liquid consonants /r/ and /l/ can form syllable nuclei in West and South Slavic languages and behave phonologically as vowels in this case. An example of a true initial cluster is the Polish word wszczniesz (/fʂt͡ʂɲɛʂ/ ("you will initiate"). In the Serbo-Croatian word opskrbljivanje /ɔpskr̩bʎiʋaɲɛ/ ("victualling") the ⟨lj⟩ and ⟨nj⟩ are digraphs representing single consonants: [ʎ] and [ɲ], respectively. Some Salishan languages exhibit long words with no vowels at all, such as the Nuxálk word /xɬpʼχʷɬtʰɬpʰɬːskʷʰt͡sʼ/: he had had in his possession a bunchberry plant. It is extremely difficult to accurately classify which of these consonants may be acting as the syllable nucleus, and these languages challenge classical notions of exactly what constitutes a syllable. The same problem is encountered in the Northern Berber languages.
There has been a trend to reduce and simplify consonant clusters in East Asian languages, such as Chinese and Vietnamese. Old Chinese was known to contain additional medials such as /r/ and/or /l/, which yielded retroflexion in Middle Chinese and today's Mandarin Chinese. The word 江, read as /tɕiɑŋ⁵⁵/ in Mandarin and /kɔːŋ⁵⁵⁻⁵³/in Cantonese, is reconstructed as *klong or *krung in Old Chinese by Sinologists like Zhengzhang Shangfang, William H. Baxter, and Laurent Sagart. Additionally, initial clusters such as "tk" and "sn" were analysed in recent reconstructions of Old Chinese, and some were developed as palatalised sibilants. Another element of consonant clusters in Old Chinese were analysed in coda and post-coda position. Some "departing tone" syllables have cognates in the "entering tone" syllables, which feature a -p, -t, -k in Middle Chinese and Southern Chinese varieties. The departing tone was analysed to feature a post-coda sibilant, "s". Clusters of -ps, -ts, -ks, were then formed at the end of syllables. These clusters eventually collapsed into "-ts" or "-s", before disappearing altogether, leaving elements of diphthongisation in more modern varieties. Old Vietnamese also had a rich inventory of initial clusters, but these were slowly merged with plain initials during Middle Vietnamese, and some have developed into the palatal nasal.
Consonant clusters occurring in loanwords do not necessarily follow the cluster limits set by the borrowing language's phonotactics. These limits are called restraints or constraints (see also optimality theory). A loanword from Adyghe in the extinct Ubykh language, psta ('to well up'), violates Ubykh's limit of two initial consonants. Also, the English words sphere /ˈsfɪər/ and sphinx /ˈsfɪŋks/, Greek loanwords, violate the rule that two fricatives may not appear adjacently word-initially.
In English, the longest possible initial cluster is three consonants, as in split /ˈsplɪt/, strudel /ˈʃtruːdəl/, strengths /ˈstɹʷeɪŋθs/, and "squirrel" /ˈskwɪrəl/, all beginning with /s/ or /ʃ/ and ending with /l/, /r/, or /w/; the longest possible final cluster is five consonants, as in angsts in some dialects /ˈæŋksts/, though that is rare (perhaps owing to the fact that it is a derivative of a recent German loanword), while final clusters of four consonants, as in sixths /ˈsɪksθs/, twelfths /ˈtwɛlfθs/, bursts /ˈbɜːrsts/ (in rhotic accents) and glimpsed /ˈɡlɪmpst/, are more common. Within compound words, clusters of five consonants or more are possible (if cross-syllabic clusters are accepted), as in handspring /ˈhændspriŋ/ and in the Yorkshire place-name of Hampsthwaite /hæmpsθweɪt/.
It is important to distinguish clusters and digraphs. Clusters are made of two or more consonant sounds, while a digraph is a group of two consonant letters standing for a single sound. For example, in the word ship, the two letters of the digraph ⟨sh⟩ together represent the single consonant [ʃ]. Conversely, the letter ⟨x⟩ can produce the consonant clusters /ks/ (annex), /gz/ (exist), /kʃ/ (sexual), or /gʒ/ (some pronunciations of "luxury"). It is worth noting that ⟨x⟩ often produces sounds in two different syllables (following the general principle of saturating the subsequent syllable before assigning sounds to the preceding syllable). Also note a combination digraph and cluster as seen in length with two digraphs ⟨ng⟩, ⟨th⟩ representing a cluster of two consonants: /ŋθ/ (although it may be pronounced /ŋkθ/ instead, as ⟨ng⟩ followed by a voiceless consonant in the same syllable often does); lights with a silent digraph ⟨gh⟩ followed by a cluster ⟨t⟩, ⟨s⟩: /ts/; and compound words such as sightscreen /ˈsaɪtskriːn/ or catchphrase /ˈkætʃfreɪz/.