Palatalization in Old English
Palatalization (global)
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Except in the cases of umlaut and vowel mutation, velar k and g tended to palatalize in Old English before front vowels and diphthongs. Here's what happened:
[k > k'> (t') > ĉ]
[g > g'> j]
The sk sound, meanwhile, became sh due to palatalization of the k, which attracted a glide from the following vowel; here are a few examples of everything so far:
cinn [ĉinn] - chin
gear [jear] - year
sceap [sheap] - sheep
fisc [fish] - fish