"Ü", or "ü", is a letter, representing a vowel, in the Estonian, Hungarian, and Turkish alphabets. It also appears in the German alphabet, where it represents "U" with umlaut, and is alphabetized together with "U". This letter also appears in Swedish for German names (where it is alphabetized together with "Y"), and in Dutch both for German names and for words that came originally from German. It is also used in writing Spanish.
In Portuguese, Spanish and Catalan "Ü" is the letter "U" with a diaeresis, indicating that it is to be pronounced in a position where it would normally be silent, specifically between a "G" and an "E" or "I" and between a "Q" and an "E" or "I" in Portuguese and Catalan.
Since in Spanish the pronunciation of "G" ordinarily changes from /g/ to /x/ (same as Spanish "J") when followed by "I" or "E", a silent "U" is added after the "G" when it is necessary to preserve the /g/ sound. For example, guerra (war) is pronounced /geRa/, not /xeRa/. If the "U" is, however, to be pronounced, then a diaeresis is added; for example, lingüística (linguistics) is pronounced /lingwistika/, not /lingistika/. Similarly in Portuguese and Catalan. In addition to this, "Ü" after another vowel in Catalan also functions like a normal U-diaresis.
In the Chinese romanizations pinyin, Wade-Giles, and the German-based Lessing-Othmer, "Ü" is used to represent the vowel of 玉 (jade) and 雨 (rain), IPA: /y/. Pinyin uses "Ü" only when ambiguity could arise, while Wade-Giles and Lessing use it in all situations.