Uppercase letter is a term used TO describe any letter in the English alphabet that is written in CAPITAL form. This includes all 26 letters of the alphabet, from A TO Z. CAPITAL letters are often used TO begin sentences, proper nouns, and abbreviations. They are also used TO differentiate between words with similar spellings, such as 'TO' and 'TO'. Uppercase letters are also used TO denote acronyms, such as NASA or FBI.