Body Jewelry is classified by shape.
These are "Straight" barbells.

	Barbells are constructed with a straight bar and usually have a ball on both ends.  There are two types classified according to how the BAR is threaded.
	Externally Threaded Barbells The threading is on the outside of the bar. The ball is internally threaded and screws on to the bar in a clockwise direction. Externally threaded is the most comoon.	Externally threaded and Internally threaded balls are removed by a counter-clockwise turning.
	Internally Threaded Barbells The threading is on the inside of the bar. The ball has an attached threaded rod and screws into the rod clockwise.	Internal is not superior because it is internal. It is only  more expensive,  more fragile and more difficult to use.
Most barbells use threading on both ends and both balls are removable.  But this is not always the case.  To reduce manufacturing cost some barbells may have one of the balls permanently attached by press-fitting and it cannot be removed.  Though there is no way of knowing which is which by visual appearance alone, the method used to removal a ball will result in the threaded ball being removed anyway.