An archaic pronoun is a pronoun that is no longer used in modern language. The two most common archaic pronouns are 'THOU' and 'THEE', which both have four letters. 'THOU' is an informal pronoun used to address one person, while 'THEE' is the objective form of the same pronoun. Both of these pronouns were commonly used in English until the 17th century, when they began to be replaced by more modern pronouns such as 'YOU'.