Knob

I opened the door for a lot of people, and they just ran through and left me holding the knob.

~ Bo Diddley

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I think that you should be careful not to trip over floorknobs. If there are doorknobs on doors and then there is a trapdoor in the floor, I suppose you could call it a trapdoorknob, but I'd argue it's a floorknob. Okay, I probably wouldn't argue it. There are more important matters in the world.

One example of a more important matter is why you're in a room with trapdoors to begin with.