developed into the palatal nasal. Origin Some consonant clusters originate from the loss of a vowel in between two consonants, usually (but not always) due to vowel reduction caused by lack of stress. This is also the origin of most consonant clusters in English, some of which go back to Proto-Indo-European times, e.g. glow from Proto-germanic *glo-, from Proto-Indo-European *gʰel-ó, where *gʰel- is a root meaning to shine, to be bright (also present in glee, gleam, glade, etc.). Consonant clusters can also originate from assimilation of a consonant with a vowel. In many Slavic languages, the combination mi- and me- regularly gave mli- and mle-. Compare Russian zemlyá with Polish ziemia, both from Proto-Balto-Slavic *źemē. Loanwords 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, ('to well up'), violates Ubykh's limit of two initial consonants. Also, the English words sphere and sphinx , Greek loanwords, violate the rule that two fricatives may not appear adjacently word-initially. English In English, the longest possible initial cluster is three consonants, as in split , strudel , strengths , and "squirrel" , all beginning with the or , containing , , or , and ending with , , or ; the longest possible final cluster is five consonants, as in angsts and twelfths in some dialects (, ), though this is rare (perhaps, for angsts, owing to being derived from a recent German loanword). However, the in angsts may also be considered epenthetic; for many speakers, nasal-sibilant sequences in the coda require insertion of a voiceless stop homorganic to the nasal. For speakers without this feature, the word is pronounced without the . Final clusters of four consonants, as in angsts and twelfths in other dialects (, ), sixths , bursts (in rhotic accents) and glimpsed , are more common. Within compound words, clusters of five consonants or more are possible (if cross-syllabic clusters are accepted), as in handspring and in the Yorkshire place-name of Hampsthwaite . 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 together represent the single