Dieresis vs. Umlaut — What's the Difference?

A dieresis is a diacritical mark consisting of two dots placed over a vowel to indicate that it forms a separate syllable from the preceding vowel, as in the word 'naïve'. Whereas, an umlaut, also involving two dots over a vowel, is used primarily in German and other Germanic languages to signal a change in the sound of the vowel, such as in 'für'.

In English, the use of a dieresis is relatively rare and mostly confined to borrowed words or proper names to ensure correct pronunciation. On the other hand, the umlaut is a fundamental aspect of German phonology and orthography, affecting both pronunciation and grammatical forms of words.

While the dieresis is used purely to clarify pronunciation without altering the original vowel sound, the umlaut not only changes the pronunciation but can also change the meaning of words, as in 'Mutter' (mother) and 'Mütter' (mothers).The visual representation of both marks is identical, but their implications and uses in language are distinctly different. Whereas the umlaut can affect the grammatical function of words, the dieresis has no impact on grammar.

In terms of historical development, the dieresis and the umlaut originated from distinct linguistic needs and traditions. While the dieresis evolved from Greek to indicate diæresis, or separation, the umlaut in German developed from a historical phonetic process where vowels were altered under the influence of the subsequent sounds.