The Scottish Vowel Length Rule is assumed to have come into being between the early Middle Scots and late Middle Scots period.
The major differences to contemporary southern English were the now well established early merger of with (dey 'die', ley 'lie'), early 15th century l-vocalisation where (except intervocalically and before ), and usually merged with , and , medial and final was lost (deil 'devil', ser 'serve').