What does it mean when the flag is half mast at Buckingham Palace?

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Tags

1. #raised
2. #actually
3. #residence
4. #standard
5. #looking
6. #sovereign
7. #signal
8. #queen
9. #never
10. #predecessor
11. #monarch
12. #which
13. #succeeds
14. #flown
15. #whenever
16. #royal
17. #immediately
18. #union
19. #means
20. #above
21. #palace
22. #flying