From what I've been told, its old cinema terminology. In the old days, when you went to the movies you not only got the main picture (in this case the "A" film) which came after the (usually shite) "B" film. In the really old days of silent film and early talkies, film companies would record any real life event (like a fire or a shooting for instance) to save money on effects and then build a film around it - hence the term a cheap "b" movie.
There you go - some really useless info for you.
Mr Mercury is indeed correct,
"...the main picture (in this case the "A" film) which came after the (usually shite) "B" film."
A trip to the cinema used to be a complete evening out. The "B" movie played first, which was followed by "Pearl & Dean" adverts, and the "coming attractions". Finally, the main or star feature, the "A" movie, would round up the evening.
I certainly recall "Cleopatra Janes" as the "B" movie to Bruce Lee's "Enter The Dragon", and Queen's "At The Rainbow" as the "B" movie to John Travolta's "Saturday Night Fever".
Anyone else remember any classic "B" cinema movies?
PS: One of my favourite classic cinema gaffs of all time was the preview to the classic softcore porn movie "Emanuelle". For those of us old enough to remember, the tagline was "Emanuelle - coming soon at a theatre near you". I bet she was!
we got "queen at the rainbow" as the "B" to pink floyd's the wall
i even recorded the rainbow on a small hand-held audio cassette recorder - about 1980 - and yes it sounded shite!
I love B movies.
And speaking of porn previews ...
At the theater here, if you go to the midnight weekend movies, they show a preview for 3D soft core porn. It's hysterical. And so early 70's!