There are a few umbrella sleeves around my pool and they have a cap that screws on and off. Frequently, there is debris or some other condition that makes the cap difficult to remove/replace and I have to get a pair of pliers or something to give me more leverage. This key fits in my umbrella sleeve caps and makes it easier to thread.

It is sized for the caps that I have. I don't know what other sizes may be available.
The diameter is 1-3/8" and the slot in the middle is 1/4" wide. This fits my caps with a little wiggle room.