'Key' in the Bible

And to the angel of the church in Philadelphia write: These things saith he that is holy, he that is true, he that hath the key of David, he that openeth and none shall shut, and that shutteth and none openeth:

And the fifth angel sounded, and I saw a star from heaven fallen unto the earth: and there was given to him the key of the pit of the abyss.

And I saw an angel coming down out of heaven, having the key of the abyss and a great chain in his hand.