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The diaeresis[a] (/dˈɛrsɪs/ dy-ERR-ee-sis; also known as the tréma[b]) and the umlaut are two different homoglyphic diacritical marks. They both consist of two dots ( ¨ ) placed over a letter, usually a vowel. When that letter is an i or a j, the diacritic replaces the tittle: ï.[1]

The diaeresis and the umlaut are diacritics marking two distinct phonological phenomena. The diaeresis represents the phenomenon also known as diaeresis or hiatus in which a vowel letter is pronounced separately from an adjacent vowel and not as part of a digraph or diphthong. The umlaut (/ˈʊmlt/), in contrast, indicates a sound shift. These two diacritics originated separately; the diaeresis is considerably older. Nevertheless, in modern computer systems using Unicode, the umlaut and diaeresis diacritics are identically encoded, e.g. U+00E4 ä LATIN SMALL LETTER A WITH DIAERESIS (HTML ä · ä) represents both a-umlaut and a-diaeresis (much like the hyphen-minus code point represents both a hyphen and often a minus sign).

The same symbol is also used as a diacritic in other cases, distinct from both diaeresis and umlaut. For example, in Albanian and Tagalog ë represents a schwa.

TeX and LaTeX

TeX (and its derivatives, most notably LaTeX) also allows double dots to be placed over letters. The standard way is to use the control sequence \" followed by the relevant letter, e.g. \"u. It is good practice to set the sequence off with curly braces: {\"u} or \"{u}.

TeX's "German" package can be used: it adds the " control sequence (without the backslash) to produce the Umlaut. However, this can cause conflicts if the main language of the document is not German. Since the integration of Unicode through the development of XeTeX and XeLaTeX, it is also possible to input the Unicode character directly into the document, using one of the recognized methods such as Compose key or direct Unicode input.

TeX's traditional control sequences can still be used and will produce the same output (in very early versions of Te

TeX (and its derivatives, most notably LaTeX) also allows double dots to be placed over letters. The standard way is to use the control sequence \" followed by the relevant letter, e.g. \"u. It is good practice to set the sequence off with curly braces: {\"u} or \"{u}.

TeX's "German" package can be used: it adds the " control sequence (without the backslash) to produce the Umlaut. However, this can cause conflicts if the main language of the document is not German. Since the integration of Unicode through the development of XeTeX and TeX's "German" package can be used: it adds the " control sequence (without the backslash) to produce the Umlaut. However, this can cause conflicts if the main language of the document is not German. Since the integration of Unicode through the development of XeTeX and XeLaTeX, it is also possible to input the Unicode character directly into the document, using one of the recognized methods such as Compose key or direct Unicode input.

TeX's traditional control sequences can still be used and will produce the same output (in very early versions of TeX these sequences would produce double dots that were too far above the letter's body).

All these methods can be used with all available font variations (italic, bold etc.).