All down through the years I have always heard single triggers as being either inertia or mechanical. While freely admitting an inertia set is, in fact, mechanical in the strictest sense I would definitely take that was the meaning. I prefer double triggers myself, but if I were to get a single trigger gun I would not want it to be inertia set, an inertia block would be fine.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra