Other Uses
A double dot is also used as a diacritic in cases where it functions as neither a diaeresis nor an umlaut. In the International Phonetic Alphabet, a double dot is used for a centralized vowel, a situation more similar to umlaut than to diaeresis. In other languages it is used for vowel length, nasalization, tone, and various other uses where diaeresis or umlaut was available typographically.
Read more about this topic: Diaeresis (diacritic)