Scottish vowel length rule

The Scottish Vowel Length Rule (also known as Aitken's law after A. J. Aitken, the Scottish linguist who formulated it) describes how vowel length in Scots, Scottish English, and, to some extent, Ulster English and Geordie is conditioned by the phonetic environment of the target vowel. Primarily, the rule is that certain vowels (described below) are phonetically long in the following environments: Exceptions can also exist for particular vowel phonemes, dialects, words, etc., some of which are discussed in greater detail below.
 * Before.
 * Before a voiced fricative.
 * Before a morpheme boundary.
 * In a word-final open syllable, save for the vowel  (or, in Geordie, ).

Phonemes
The underlying phonemes of the Scottish vowel system (that is, in both Scottish Standard English dialects and Scots dialects) are as follows:

★ = Vowels that definitively follow the Scottish Vowel Length Rule.

Rule specifics and exceptions
The Scottish Vowel Length Rule affects all vowels except the always-short vowels 15 and 19 ( and ) and, in many Modern Scots varieties, the always-long Scots-only vowels 8, 11, and 12 (here transcribed as, and ) that do not occur as phonemes separate from  in Scottish Standard English. The further north a Scots dialect is from central Scotland, the more it will contain specific words that do not adhere to the rule.


 * and (vowels 15 and 19) are usually short in all environments.
 * In some Modern Scots varieties may merge with  in long environments. In Ulster Scots,  and  are usually always long and the  realisation of  is short before a voiceless consonant or before a sonorant followed by a voiceless consonant but long elsewhere.
 * ,, , , , , and ,(vowels 2, 4, 5, 6, 7, 13, and 14) are usually long in the following environments and short elsewhere:
 * In stressed syllables before voiced fricatives, namely, and also before . In some Modern Scots varieties, before the monomorphemic end-stresses syllables , + any voiced consonant,  and . In Shetland dialect the  realisation of underlying , usual in other Scots varieties, remains a long environment.
 * Before another vowel and
 * Before a morpheme boundary so, for example, "stayed" is pronounced with a longer vowel than "staid".
 * (vowel 12) usually occurs in all environments in final stressed syllables.
 * Vowel 8a, which only occurs stem-finally, and vowel 10 are always short; therefore, vowel 1 in its short form (according to the Rule), vowel 8a, and vowel 10 all merge as the diphthong . In its long form, vowel 1 is here transcribed as.

History
The Scottish Vowel Length Rule is assumed to have come into being between the early Middle Scots and late Middle Scots periods.