Bars are available in different models, both completely straight (for tongue piercing, breast piercing, etc.), curved (for eyebrow piercing, intimate piercing, etc.) and with a plate on one side for the lip.
You measure a bar by taking the measurement between the balls, just the bar itself. Neither balls nor threads should be included.
There are two options when it comes to threading on jewelry: Internally threaded and externally threaded.
By internally threaded jewelry is meant that the rod is hollow and the ball has a threaded pin that is threaded into the rod. On externally threaded jewelry, the thread sits on the rod and is screwed into the hollow ball.
The most common are externally threaded jewelry, at least so far, many studios have started to switch to internally threaded jewelry. The range of externally threaded jewelry is much larger and even cheaper.