Diaeresis
With a diacritic, one vowel is sounded separately from its neighbour. So, in the word zöology, it is pronounced as a long 'o' followed by a short 'o'. Without the diaeresis, it might be pronounced like the word 'zoo'. It is sometimes described as "two vowel sounds occurring in adjacent syllables". It is often used in names where the last vowel is sounded, as in Chloë or Brontë.