U with diaeresis

German umlauts began as a small e written above the vowel. In Kurrent handwriting an e was made of two short strokes, so the raised e came to look like two dots, and printers adopted the dots from the sixteenth century.

Spanish uses the same two dots for another purpose. In pingüino and cigüeña they show that the u is pronounced, which it would not be in plain gue or gui.