In the 40s and early 50s, it was all double features, except for the occasional Gone With The Wind-type exception. Two movies played Monday through Wednesday, and two Thursday through Sunday (actually, I can't remember which pair got Thursday). There would be an A picture and a B picture, sort of like the A and B sides on pop records. Some folks would just come for the A show and then go home. The A show always played in the prime-time slot.
Later, when TV took over and theaters began running single features, grindhouses ran low-budget and second-run movies, and nobody was going to pay to see just one of those. In fact, in the end, most of them were running three or four features at a time.
Or so I remember it.