The Diphthong */eu/
In the West Germanic variety that gave rise to Old English, a-mutation did not affect the second element of the diphthong */eu/ (for which the earliest Old English texts have eu): treulesnis "faithlessness", steup- "step-" (Epinal Glossary 726, 1070); but in other branches of West Germanic */eu/ became */eo/ unless followed by */i/ or */w/, e.g. Old Saxon breost "breast" vs. treuwa "fidelity". In Old Norse, */eu/ → /ju:/, without regard to a-mutation, e.g. Old Icelandic djúpr. In Old West Norse, the second element of the diphthong was lowered before a dental or alveolar consonant or /m/, transforming the diphthong to /jo:/, e.g. Old Icelandic bjóða "to offer". In Old East Norse, /ju:/ was simplified to /y:/ after post-consonantal /r/ as early as the 10th century (and later after initial /r/ and post-consonantal /l/), e.g. brýtæ vs. Old Icelandic brjóta.
Read more about this topic: Germanic A-mutation