What exactly is an umlaut?

Sure, an umlaut is those two dots over a letter. But what exactly is it? What does it mean, and what started it?

My old video about the umlaut:
   • The umlaut  

Chapters:
00:00 When an umlaut isn't an umlaut
01:46 The vowel chart
02:58 Writing the sounds
04:02 Umlautable vowels
05:08 Performing the umlaut
05:35 How it all started

Music:
"Style Funk" and "Hot Swing"
by Kevin MacLeod https://incompetech.com/
Creative Commons Attribution licence

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