In the development of German, the vowel sounds in some words shifted, often a back vowel moving to a front position when pronounced. Such a change of sound is referred to as umlaut (German, um – around, Laut – sound). In German, a special diacritical mark [¨] is used to represent the change of the marked vowel, as in Fuß > Füße (foot > feet). Originally, the umlaut change was indicated by inserting an e after the affected vowel, such as in Mueller (miller) or by writing a small e above the vowel. By the 16th century, this scribal practice was beginning to appear in typeset text. Here is an example of a certificate from 1799 and a detail of the small superposed e to indicate the umlaut.
Here is a detail from the main heading.
Eventually, the thicker vertical strokes of the e were reduced to the two dots of the diacritical mark we now recognize as the German umlaut.