Ë

From Teflpedia

Ë (E-diaeresis, lower case ë) is the letter E with a diaeresis diacritic.

In English, this is used sometimes to indicate the vowel should be sounded, e.g. Brontë. Otherwise, it's used in French loanwords, e.g. noël, and some names, e.g. Zoë.