◌̈

Used to indicate that two identical vowels occur in separate morphemes and are pronounced separately: coöperate, reënter, reëstablish, noöne, reöpen.

Used to indicate that the marked vowel is the nucleus of its own syllable: Boötes, Brontë, Chloë, Zoë, (dated) aëroplane, aërial.

Retained in foreign (mostly French) loan words where vowels are pronounced separately: naïve (or naive), Noël (or Noel), but also for umlaut in German Götterdämmerung, Führer, and terms derived from given names, such as Möbius strip