Why are B movies called B movies?

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Tags

1. #hollywood
2. #feature
3. #precisely
4. #double
5. #sides
6. #recorded
7. #films
8. #motion
9. #golden
10. #movie
11. #distribution
12. #budget
13. #during
14. #bottom
15. #commercial
16. #original
17. #music
18. #arthouse
19. #intended
20. #usage
21. #picture
22. #identified
23. #publicized