Ü, or ü, is a character which typically represents a close front rounded vowel [y]. It is classified as a separate letter in several extended Latin alphabets, or alternatively as the letter U with an umlaut/diaeresis in others.
Letter Ü 
The letter Ü is present in the Hungarian, Karelian, Turkish, Uyghur Latin script, Estonian, Azeri, Turkmen, Crimean Tatar and Tatar Latin alphabets, where it represents a close front rounded vowel [y]. It is considered to be distinct letter, collated separately, and not considered a simple modification of U or Y. It is distinct from "UE".
This same letter appears in the Chinese romanizations pinyin, Wade-Giles, and the German-based Lessing-Othmer, where it represents the same sound [y] e.g. 玉 (jade) or 雨 (rain). Pinyin uses Ü only when ambiguity could arise with similarly romanised words containing a U, whereas Wade-Giles and Lessing use Ü in all situations. As the letter "ü" is missing on most keyboards and the sound "v" is not present in standard Mandarin, the letter "v" is used on most computer Chinese input methods to enter the letter "ü". As a result, romanization of Chinese with the letter "v" representing the ü sound is sometimes found.
An identical glyph, U with umlaut, appears in the German alphabet. It represents the umlauted form of u, which results in the same sound as the letter Ü mentioned in the previous section: [y]. The letter is collated together with U, or as UE. In languages which have adopted German names or spellings, such as Swedish, the letter also occurs. It is however not a part of these languages' alphabets. In Swedish the letter is called tyskt y which means German y.
In other languages that do not have the letter as part of the regular alphabet or in limited character sets such as ASCII, U-umlaut is frequently replaced with the two-letter combination "ue". Software for optical character recognition sometimes sees it falsely as ii.
In Spanish it is used to distinguish between "gue"/"güe" and "gui"/"güi": antigüedad (antiquity), pingüino (penguin). This is also the case for the Nicaraguan demonym, Nicaragüense. Similarly in Catalan language,
- "gue~güe" are [ɡe]~[ɡwe],
- "gui~güi" are [ɡi]~[ɡwi],
- "que~qüe" [ke]~[kwe],
- "qui~qüi" [ki]~[kwi],
as in aigües, pingüins, qüestió, adeqüi. Also ü is used to mark that vowel pairs that normally would form a diphthong must be pronounced as separate syllables, examples: Raül, diürn.
In French, the diaeresis appears over the "u" only very rarely in some uncommon words, capharnaüm [-aɔm] ('shambles'), Capharnaüm/Capernaüm [-aɔm] or Emmaüs [-ays]. After the 1990 spelling reforms, it is applied to a few more words, like aigüe (formerly aiguë), ambigüe (formerly ambiguë) and argüer [arɡ(ɥ)e] (formerly without the diaeresis).
Usage in phonetic alphabets 
Historically the unique letter Ü and U-diaeresis were written as a U with two dots above the letter. U-umlaut was written as a U with a small e written above: this minute e degenerated to two vertical bars in medieval handwritings. In most later handwritings these bars in turn nearly became dots.
In modern typography there was insufficient space on typewriters and later computer keyboards to allow for both a U-with-dots (also representing Ü) and a U-with-bars. Since they looked near-identical the two glyphs were combined, which was also done in computer character encodings such as ISO 8859-1. As a result there was no way to differentiate between the three different characters. While Unicode theoretically provides a solution, this is almost never used.
Computing codes 
|Unicode name||LATIN CAPITAL LETTER
U WITH DIAERESIS
|LATIN SMALL LETTER
U WITH DIAERESIS
|UTF-8||195 156||C3 9C||195 188||C3 BC|
|Numeric character reference||Ü||Ü||ü||ü|
|Character entity reference||Ü||ü|
|EBCDIC code pages||252||FC||220||DC|
|DOS/OEM code pages||154||9A||129||81|
|Windows/ANSI and ISO-8859 code pages||220||DC||252||FC|
|Macintosh Roman and Central European||134||86||159||9F|
The methods available for entering ⟨Ü⟩ and ⟨ü⟩ from the keyboard depend on the operating system, the keyboard layout, and the application.
- Microsoft Windows – some keyboard layouts feature separate keys for ⟨Ü⟩
- Using the Swiss French keyboard, ⟨ü⟩ can be entered by typing «Shift+È»
- Using the US International layout, ⟨ü⟩ can be entered by typing «AltGR+Y»
- Microsoft Windows: with the Number Lock on, hold down the Alt key while typing on the numeric keypad the decimal value of the code point from the active DOS/OEM code page without a leading zero, then release the Alt key; i.e. «Alt#154» for ⟨Ü⟩ and «Alt#129» for ⟨ü⟩
- Microsoft Windows: with the Number Lock on, hold down the Alt key while typing on the numeric keypad the decimal value of the code point from the active ANSI code page with a leading zero, then release the Alt key; i.e. «Alt#0220» for ⟨Ü⟩ and «Alt#0252» for ⟨ü⟩
- Microsoft Word for Windows: type «Ctrl+:, Shift+U» for ⟨Ü⟩ or «Ctrl+:, U» for ⟨ü⟩
- Mac OS with an English keyboard layout (Australian, British, or US): type «Option+U, Shift+U» for ⟨Ü⟩ or «Option+U, U» for ⟨ü⟩
See also 
Letter U with diacritics
Letters using umlaut or diaeresis sign ( ◌̈ )