I'll take a stab at it.   "Used to" has been commonly used as a semi-auxiliary since the 14th c.  There are a few citations earlier in the OED of "use" with a <t> suffix, which might suggest a voiceless pronunciation, but these were not in the past habitual sense.  What I suspect happened is that as "used to" began to grammaticalize into the auxiliary system in the 14th c. it underwent the lost of lexical meaning and the phonological reduction that are common in grammaticalization.  Compare, for example, the reduction of "have" to "'ve" or just a schwa in its auxiliary use but not in its lexical uses, at least for American English.  The phonological reduction of "used to" involves the reduction of "to" to unstressed /t@/, where /@/ represents schwa.  The /d/ disappears before the /t/, perhaps by assimilation and then deletion, giving us [log in to unmask]  Grammaticalization is a broad term for what's happened, but probably broader than you wanted.

Herb
-----Original Message-----
From: Assembly for the Teaching of English Grammar [mailto:[log in to unmask]] On Behalf Of Brett Reynolds
Sent: Tuesday, July 06, 2010 8:56 AM
To: [log in to unmask]
Subject: Re: NOT A PAST PERFECT QUESTION
Importance: Low

On 2010-07-06, at 8:27 AM, Assembly for the Teaching of English Grammar wrote:

> Is there a special terminology that would refer to the s/z sound
> distinction in the word 'used' in the following sentences:

/z/ is voiced and /s/ is unvoiced. Is that what you mean?

Best,
Brett

-----------------------
Brett Reynolds
English Language Centre
Humber College Institute of Technology and Advanced Learning
Toronto, Ontario, Canada
[log in to unmask]

To join or leave this LISTSERV list, please visit the list's web interface at:
     http://listserv.muohio.edu/archives/ateg.html
and select "Join or leave the list"

Visit ATEG's web site at http://ateg.org/

To join or leave this LISTSERV list, please visit the list's web interface at:
     http://listserv.muohio.edu/archives/ateg.html
and select "Join or leave the list"

Visit ATEG's web site at http://ateg.org/