"Michael" wrote in news:F_fXa.59639$o27.1329609
@twister.rdc-kc.rr.com:
My question: the 90 mm cassette doesn't correspond to
the sizes I see advertised. I see a cassette of 11-28,
12-21, for example, but I don't see how these relate to
90mm.

A 12-21 cassette has a small cog with 12 teeth and a large cog with 21 teeth.
The number of teeth should be stamped somewhere on each cog, or you can just
count them.