Scottish Vowel Length Rule

The Scottish Vowel Length Rule, also known as Aitken's law, states that vowels tend to be long before the alveolar approximant, voiced fricatives and a morpheme boundary, but short elsewhere. Unlike in standard English, vowels are not long before voiced plosives. The rule was proposed by the Scottish linguist A.J. Aitken. Vowel length is conditioned by the phonetic environment of the target vowel, but the environment is more restricted than in standard English.

In standard English, the vowel of seed is longer than in seat. However, this doesn't apply to Scottish English, which maintains the same vowel length. The words leaf and leave have a difference in vowel length because leave ends with a voiced fricative.

The words greed and agreed have different vowel lengths. Though they both end with a voiced alveolar plosive, the word agreed has a morpheme boundary. The morphological structure can be represented as follows:

agree + (e)d = agreed

Whether the past tense marker ed was first suffixed and the vowel then deleted, or deletion occurred at the moment of suffixation is a matter of speculation. In any case, agreed consists of two morphemes. For this reason, agreed has a longer vowel than greed.

The Scottish Vowel Length Rule applies to the pronunciation of vowels in Scottish English. The rule is not categorical for all Scottish speakers. In the northern parts of Scotland, in particular, many specific words do not adhere to the rule. Nevertheless, the rule makes clear that vowels in Scottish English are pronounced differently than in other varieties of English.