Changing fluorescent tube: how many degrees to turn?
The bottom two are bi-pin fluorescent tubes. There are two types of bi-pin socket you will encounter on straight tube T5, T8 and T12 fluorescent lights.

This type requires 90 degree rotation.

Removal can be a real pain the first time you do it as the pins can weld to the brass contacts and you will end up breaking the socket if you get rough with them. I use silicone spray with a straw to shoot some into the socket when I encounter one that's semi-spotwelded itself in. It's non-conductive and prevents future issues. Rotate the tube 90 degrees so they align with the slot and pull out. Also, if the tube is too long to allow it to drop out of the fixture opening, there often will be a slot in one end to drop the pins into so the other end will clear the opening.

Align the pins vertical, insert the tube until it stops and rotate until you feel both pins click in place, it is possible to have only one set engage in the indent in the brass contacts that are visible.



This type pushes straight into the socket and removal is straightforward.



The top two tubes in that picture are compact fluorescent and pull straight out of the socket.