Plugs and Flesh Tunnels Plugs are a favorite for people to wear in larger gauged piercings. These are measured much the same as a barbell - from one end to the other with some room left for it to fit snugly. There are non-flared plugs, which require you to wear an o-ring in the front and back to keep it from falling out. There are also single flared plugs, which have a front flare and require an o-ring in the back. Then there are double flared plugs, which have a flare on the front of back.
To insert a double flared plug it is recommended to gently rock it in, as well as back out, and soaking it beforehand may greatly help (but not recommended with organics as water can damage them).
Flesh tunnels are essentially the same, only they are hollow in the middle so you can see through the jewelry.
Plugs and flesh tunnels come in acrylic, titanium, stainless steel, glass, and organic materials such as wood and bone.