Thanks as always for the quick reply. I'm back up on Mavericks.
Disaster may be a bit dramatic, but I could not get Sierra to log in with my primary user account, which is where all my photos, documents, music, etc. was, and I couldn't find an easy solution on the Web. Combine that with general sluggishness and then an inexplicable system freeze, and I decided I'd seen enough. Maybe a 1.8 GHz MacBook Air just doesn't have the horsepower to drive Sierra. I don't know. I'll probably just wait until I buy a new laptop loaded with Sierra or its successor to make the transition.