Well, I see this: http://www.jhiggins.net/sf_proddetai...181C0C0OQSP9FD

It claims to be 100% wool twill in both the front and back.

I can't readily find any others that even mention or show what the back is made of. If silk is the norm, and is socially acceptable when worn without a jacket, that's cool; I won't worry about it. I just didn't know if it was proper. I've never owned a waistcoat, nor do I know anyone who wears one. I thought someone had told me long ago that the silk back was not meant to be seen, but maybe I just imagined it.