Middle Chinese *z- usually becomes Mandarin x- before palatals and s- elsewhere, but Middle Chinese 囚 *zu 'prisoner' became Mandarin qiu rather than *xiu. q- implies Middle Chinese *dz-. *dz-variants of *z-words also underlie Mandarin 詞辭 ci (instead of *xi).

Was the affrication of *z- a trait of a single Middle Chinese dialect? I don't think so, since the trait is not unique to one region and there is variation within branches (Mandarin, Wu, Min).

*dz- for 囚

[tɕʰ]: Beijing, Jinan, Hankou, Yangzhou, Nanchang

but Xi'an and Chengdu have [ɕ] < *z-

[tɕ]: Changsha

[tsʰ]: Cantonese, Chaozhou, Fuzhou

[dʑ]: Wenzhou, Shuangfeng

*z- for 囚

[ɕ]: Xi'an and Chengdu

[s]: Meixian, Xiamen

[z]: Suzhou (8.26.3:52: [z] could be from *dz-, since Suzhou has [z] but not [dz])

The patterns of distribution for initials descended from *dz- and *z- are similar for 詞辭 with the following exceptions:

Chengdu and Meixian have [tsʰ] < *dz-

Chaozhou and Fuzhou have [s] < *z-

Wenzhou has [z] < *z-

*z- and *dz- are phonetically similar, so it is likely that they were independently confused by speakers of more than one dialect.

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