ASSIBILATION

During EME, the palatal semivowel EME /j/ was sometimes inserted before the unstressed vowel of a syllable following the stressed syllable. Thus, e.g., EME /presur/ pressure became EME /presjur/; EME /grædu'l/ gradual became EME /grædju'l/.

If the consonant preceding the intrusive /j/ was an /s/, /z/, /t/, or /d/, the consonant combined with the /j/ to produce a palatal fricative or affricate. This phenomenon is called "assibilation." Assibilation produced the following results in EME.

/dj/
>
/j</
/sj/
>
/s</
/tj/
>
/c</
/zj/
>
/z</

Assibilation is the origin of the phoneme /z</ in EME.