Sentence:  <WSD>Acetates</WSD> are a relic of the days before cassettes, DAT, and recordable CDs came into widespread use in recording studios. Manufactured from aluminum, and coated in a thin sheet of vinyl, they were produced to allow the concerned parties to hear how a particular version of a recording would sound outside the studio, on their home hi-fi, for example. 
Meaning of the word marked with <WSD> tags:  (acetate disc): a disc of aluminium covered in a wax used to make demonstration copies of a phonograph record.