The voicing of fricatives in the margins of weak syllables

In Early Modern English the voicing of /s/ in weak syllables was not perfected, yet. A syllable is a unit that consists of onset, nucleus and coda. Weak syllables are syllables without stress, whereas strong syllables are stressed.

In the English language it is possible to have a voice contrast in fricatives after sonorants.

Fricatives are consonant sounds. These sounds are produced through such a narrow air flow in the vocal tract, that the sound seems to be created through friction. Fricatives are [f], [v], [s], [z], and some other sounds. Sonorants describe a class of sounds that includes vowels, glides, liquids, nasals and nonobstruents. They are produced by using relatively free airflow through the mouth or the nose. Some of them are: [l], [w], [r], [n].

Recapitulating you can say that there was no problem to use /s/-endings after the sonorants /n, l, r/. E.g.: heel - heels

Therefore it was a problem, when there was a voiced obstruent used with /s/. Obstruents build a sound class that contains nonnasal stops, fricatives and affricatives. Stops are sounds that completely stop in the mouth for a short moment, as there are [p], [t], [k], [b], and some more. Nonnasal means that stops as [n] for example are left out. For the explanation of fricatives look above (“The two Middle English changes”). Affricatives are sounds ([č]) that are formed by a little stop that is followed by a release characteristic of a fricative. Plural forms that were not allowed to use were for example <tungs> and <selvs>.

The deletion of certain weak vowels

The second change that took place was the deletion of weak vowels in plural and genitive endings. However these deletions were uncommon and only occurred in verse. This inconsistency shows that the {-s} suffix system was a still altering system in the end of the sixteenth century.