Not an umlaut, a diaeresis - it shows that the letter has its own sound (unlike the word "coop", for example). You'd generally use it for proper nouns (like Br

Not an umlaut, a diaeresis - it shows that the letter has its own sound (unlike the word "coop", for example). You'd generally use it for proper nouns (like Brontë or Boötes) and rare loanwords - it doesn't seem very useful for common dictionary words, since anyone who can understand the word itself probably knows how to pronounce it.