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[1] and Geordie[2] 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.
The underlying phonemes of the Scottish vowel system (that is, in both Scottish Standard English dialects and Scots dialects) are as follows:[3] ★ = Vowels that definitively follow the Scottish Vowel Length Rule.
[17] The further north a Scots dialect is from central Scotland, the more it will contain specific words that do not adhere to the rule.
[18] The Scottish Vowel Length Rule is assumed to have come into being between the early Middle Scots and late Middle Scots periods.