The Umlaut

Pronunciation of Indo-European languages underwent widespread changes in the Middle Ages. One of those changes is known as the Great Vowel Shift, in which the vowel sounds of words were “fronted” (moved forward in the mouth). Muller became Mueller. In German, this was called "umlaut," meaning "around," as the new sound was close to the original sound. Scribes at that time often indicated this by writing an ‘e’ above the vowel. Eventually, the ‘e’ was reduced to two dots, which is what an umlaut looks like today. Here is a certificate from 1799, well after the advent of printing, in which the practice was still used.