The letter "ä" is a character used in several languages, including German, Swedish, and Finnish. It represents a vowel sound that is similar to the English "a" in "cat." In German, "ä" can appear in words like "Mädchen" (girl) and changes the pronunciation of the base vowel "a." In Swedish, "ä" is considered a separate letter in the alphabet, following "z." It is pronounced like the "a" in "bad." The letter is also used in Finnish, where it appears in words such as "käsi" (hand). Each language has its own rules for using "ä."