Diairesis

Diæresis (sometimes written as "diaeresis") is a diacritical mark that consists of two dots placed over a vowel. In English, it is often used to indicate that two adjacent vowels should be pronounced separately rather than as a single sound. For example, in the word "naïve," the diæresis over the "i" indicates that it should be pronounced distinctly from the "a" rather than creating a diphthong.