Independent letter 
The letter Ä occurs in the Swedish, Finnish, Skolt Sami, Karelian, Estonian, Luxembourgish, North Frisian, Saterlandic, Emiliano-Romagnolo, Rotuman, Slovak, Tatar, and Turkmen alphabets, where it represents a vowel sound. In Finnish and Turkmen this is always [æ]; in Swedish and Estonian regional variation, as well as the letter's position in a word, allows for either [æ] and [ɛ]. In Slovak Ä stands for [ɛ] (or a bit archaic but still correct [æ]).
In the Nordic countries, the vowel sound [æ] was originally written as "Æ" when Christianisation caused the former Vikings to replace the Runic alphabet with the Latin alphabet around A.D. 1100. The letter Ä arose in German and later in Swedish from originally writing the E in AE on top of the A, which with time became simplified as two dots. In the Icelandic, Faroese, Danish and Norwegian alphabets, "Æ" is still used instead of Ä.
Finnish later adopted the Swedish alphabet during the 500 years that Finland was part of Sweden. Although the phenomenon of Germanic umlaut does not exist in Finnish, the phoneme [æ] does. Estonian similarly gained the letter via Germanic influence.
The letter is used in some Romani alphabets as well.
A similar glyph, A with umlaut, appears in the German alphabet. It represents the umlauted form of a, resulting in [ɛ] (or [e] for many speakers). In German, it is however called "Ä" (pronounced [ɛ]) or "Umlaut-A", not "A-Umlaut" (which would refer to "Germanic a-mutation"). With respect to diphthongs, Ä behaves as an E, e.g. Bäume /bɔɪmə/ (Engl.: trees). In German dictionaries, the letter is collated together with A, while in German phonebooks the letter is collated as AE. The letter also occurs in some languages which have adopted German names or spellings, but is not a part of these languages' alphabets. It has recently been introduced in revivalist Ulster-Scots writing.
The letter was originally an A with a lowercase e on top, which was later stylized to two dots.
Phonetic alphabets 
- In the International Phonetic Alphabet, ä represents an open central unrounded vowel (in distinction to an open front unrounded vowel).
- in the Rheinische Dokumenta, a phonetic alphabet for many West Central German, the Low Rhenish, and few related languages, "ä" represents the sound [ɛ].
Historically A-diaeresis was written as an A with two dots above the letter. A-umlaut was written as an A with a small e written above: this minute e degenerated to two vertical bars in medieval handwritings (A̎ a̎). In most later handwritings these bars in turn nearly became dots.
Æ, a highly similar ligature evolving from the same origin as Ä, evolved in the Icelandic, Danish and Norwegian alphabets. The Æ ligature was also common in Old English, but had largely disappeared in Middle English.
In modern typography there was insufficient space on typewriters and later computer keyboards to allow for both A-diaeresis (also representing Ä) and A-umlaut. 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 different characters. Unicode theoretically provides a solution, but recommends it only for highly specialized applications.
|Look up ä in Wiktionary, the free dictionary.|
- Unicode FAQ Characters and Combining Marks – "Unicode doesn’t seem to distinguish between trema and umlaut, but I need to distinguish. What shall I do?"
Letter A with diacritics
Letters using umlaut or diaeresis sign ( ◌̈ )