[General] turn Queen's or King's evidence

[thedaffodils]

Hello! I have understood the meaning of this idiom-turn Queen's or King's evidence. It has another variation -turn state's evidence. I am clear about the choose of "Queen", "King" or "State". But I have two questions about it as below:

• Why is "turn" used in this idiom? What does it mean?
• Why is "Queen's" evidence but not police's evidence?

Thanks!

P.S. The definition of this idiom.
Turn state's evidence - Wikipedia, the free encyclopedia

[Raymott]

Originally Posted by thedaffodils

Hello! I have understood the meaning of this idiom-turn Queen's or King's evidence. It has another variation -turn state's evidence. I am clear about the choose of "Queen", "King" or "State". But I have two questions about it as below:

• Why is "turn" used in this idiom? What does it mean?
• Why is "Queen's" evidence but not police's evidence?

Thanks!

P.S. The definition of this idiom.
Turn state's evidence - Wikipedia, the free encyclopedia

1. The article talks about criminals who decide to give evidence against their fellow crims in exchange for leniency or some benefit. They have "turned" in that they have changed sides from obstructing police to helping police.
2. In a criminal trial, it is the state that prosecutes. Obviously, the police give evidence for the state (since they are employees of the state), but other people also give evidence one way or the other. Any deals done by the criminals are done with the state prosecutors, perhaps via the police.

[thedaffodils]

Hello Raymott,

Thank you for your elucidation. They are clear now.