In Early Modern English, there was a number of morphemes which carried essentially the same meaning. For example, the plural markers < -en> and < -s> were sometimes interchangeable, as in shoon or shoos (shoes). One such pair of allomorphs are the second person possessive adjectives thy and thine, which occurred in complementary distribution.

Before the regularization resulting in today’s form your, Early Modern English had two forms, thy and thine, both of which functioned as second person possessive adjectives, as in thy beautiful face and thine eyes. We might immediately think that the distinction is analogous to present day my and mine. However, it is not the case. Although in Early Modern English the two pairs of possessives behaved in the same manner, as in I saw it with mine own eyes, today the possessive adjective my occurs before a noun and has the role of determiner. On the other hand mine is a proper possessive pronoun which stands freely in a sentence.

In actual fact, the case appears to be that the case of the thy/thine allomorphs is identical to the a/an distinction, i.e. the difference lies in the pronunciation of the two allomorphs. That is to say that in Early Modern English, the use of the two forms was dependent on the phonetic environment, specifically the following speech sound. Therefore thy could be expected to appear before words beginning with a consonant, whereas thine occurred before vowels.