The term 'ADDITION TO A LETTER' can refer to two different types of typographical marks. The first is a serif, which is a small line or stroke added to the end of a letter in certain typefaces. Serifs are used to make text easier to read and are often found in traditional print media. The second is a cedilla, which is a diacritical mark that looks like a comma placed beneath the letter 'c' in some languages. It is used to indicate a change in pronunciation and is most commonly found in Spanish and Portuguese.