A shirt garter is a piece of clothing associated with the military, and serves a purpose similar to the garter (and garter belt) in a female's wardrobe. There are two key differences: no garter belt is required, and instead of holding one's stockings up, it holds one's shirt down. Shirt garters are responsible for that super-tight tuck of a Marine's shirt, right after he stands up. Shirt garters keep a button-down shirt from puffing out at the back, bunching up on the sides, or becoming in any way wrinkled. Shirt garters are common in all branches of military service.
How it works
It works like this: at the end of the elastic shirt garter--just like on the female model--there is a metal hoop and a loosely-fitted rubber stud assembled into a clip. The clip is secured by slipping a layer of cloth against the hoop and then locking the rubber stud into the hoop. This causes the clip to tightly grip the cloth. Each shirt garter is comprised of two such clips at each end of a long elastic garter; some models have a single clip at the sock end and a Y-shaped split with two clips at the shirt end.
Wearing shirt garters
Put on your shirt and socks, but not your pants. If you're using straight shirt garters, you'll need four. Two attached to the front (about midway across your thigh) and two in back (evenly spaced). If you're using Y-garters, clip them to the sides of your shirt. Once they're attached to your shirt, adjust their length (the tab on them works like suspenders or a bra strap) so all four are even, within a few inches of the tops of your socks. With Y-garters, attach them to the outside of your socks. With straight garters, go front-and-back, either shirt-front to sock-front or criss-crossed around the outside of the knee. You'll notice that your shirt is now pulled tightly down your chest and back. It will stay there until you unclip the garters.
Now: put on your pants.
The Pros and Cons
The benefits are clear: a tight, crisp military tuck. Combined with diligent starching and ironing, your shirt will be flat and smooth all day, even if you lay down and take a nap under your desk during lunch. The downsides include missing leg hair (from the elastic contracting and expanding), sore shoulders (from the tension of the shirt being pulled down all day long), and of course, looking extremely silly if you have to take off your uniform in front of anyone.
Hey! I said put on your pants.