breaking
In the history of the English and of some other languages) the
change of a vowel to a diphthong under the influence of a
follwoing consonant or combination of consonants; e.g., in
Old English, the change of an -a- to -ea- and
of -e- to -eo before preconsonantal -r-
or -l- and before -h-, as in earm (arm)
developed from arm and eorthe (earth) from erthe.