The letter "U" is the 21st letter of the English alphabet. It is a vowel and is often used to represent the sound /juː/ as in "unicorn" or /ʌ/ as in "cup." In the English language, "U" can also function as a pronoun, referring to the person being addressed. In mathematics, "U" is commonly used to denote a union of sets, represented as A ∪ B. In music, "U" can refer to the note in the solfège system, where it is often associated with the sound of "do" or "re."