Ferrules are a fairly basic piece of hardware once you look into them; they are simply small metal “cups” which hold the ball end of the string in place as it is fed through the back of the guitar.

The advantages compared to a “string though top” setup are pretty wide ranging, but the main attraction is that they allow the strings movement to resonate more efficiently through the wood of the guitars body, giving a much more honest tone to the sound, really emphasising that classic Tele® snap and twang that has proven so popular in recent decades.

The Knarled Edge ferrules turn up fairly often on guitar made in the far east rather then in the USA (which seem to use the Smooth edged version with more regularity) - exactly the same principle but the knarlled edge have "teeth" around the barrel section to help them grip the body more efficiently.

These "bullet" ferrules are a slight departure from the original version of the 50s and 60s. They initially debuted as a companion part to a set of strings in the mid 70s and more commonly seen on hard tails.