How did the Old Scandinavian vowel system change over time?

The Old Scandinavian vowel system originally comprised nine vowels—each capable of being long, short, or nasalized—along with three falling diphthongs. Over time, this system underwent significant transformations:1

• Nasalization and Diphthongs: Nasalized vowels were lost, and in the 10th century, diphthongs shifted to long vowels in Danish and Swedish.1
• Vowel Shifts and Mergers: Short low umlauted vowels coalesced with neighboring vowels. Long ā was rounded to å, while Norwegian and Swedish saw rounded vowels shift upward and forward. Additionally, high round vowels merged with unround vowels in Icelandic and Faroese.1
• Unstressed Vowels: While Icelandic and Faroese preserved original unstressed vowels, other languages saw partial or complete mergers, often resulting in the schwa sound, or total loss in certain dialects.1
• Quantity and Diphthongization: A late Middle Ages quantity shift lengthened short vowels before single consonants and shortened long vowels before clusters. Conversely, many dialects and standard languages later underwent diphthongization of long vowels.1