As with normal progression, the barbells started to be offered in different colors. The better quality would be in an anodized titanium (so that it would not loose it's color) and lower quality barbells were titanium plated, but the color would eventually wear away.
Soon customers were able to buy barbells with a decoration on the bar, such as a star. If the customer stayed with a small design, they were able to put the barbell into their piercing. However, as the designs became larger there was not enough room to slip the barbell into place, and some designs just could not be worn.
Today the designs are made independent of the barbell. These are made to slip onto the bar, so that you can wear a steel, titanium or gold barbell & interchange your design.
To put these new style designs onto the barbell, they can slip all the way to one end. This allows the wearer to select bigger designs and still be able to get the jewelry into the piercing.
The newest designs also have a rubber setting, to snug the barbell and stay in the spot it has been placed.
For more example of slide on designs visit Industrial Jewelry