... be understood as French or English z, not in its German use. Strictly speaking, this "z" (intervocalic -s-) was not voiced but was a soft voiceless sound, a sibilant intermediate between our s and z. In modern North German it has become voiced to z. It is important not to confound this s—z with the voiceless intervocalic s that soon ... — Language - An Introduction to the Study of Speech • Edward Sapir
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