The Scots Vowel-Length Rule, also known as Aitken's Law after Professor A.J. Aitken who formulated it, describes how vowel length in Scots and Scottish English is conditioned by environment. (Phonetics in IPA.) The rule affects all vowels in Central dialects, while in peripheral dialects some vowels remain unaffected.

• and [a] are usually short.
• [e], [i], [o], [u] and [ø] are usually long:
  • in stressed syllables before [v], [ð], [z], [ʒ] and [r].
  • before another vowel and
  • before a morpheme boundary.
• [ɑ], [ɒ] and [ɔ] are usually long in most dialects.
• The diphthong [əi] usually occurs in short environments and [aɪ] in the long environments described above.