I spent 3 years combine playing professional football in the NFL and USFL and I've always seen players use both methods. Its your choice to what seems more comfortable to you.
I only played for four years in High School, and I wore regular underwear under mine. I knew guys who used their girdle as underwear, too. Like the first answerer said, it's your choice. Now, it seems some NFL players choose "Neither" (I was watching a Detroit Lions game earlier this year and their running back had a large hole in the back of his pants, and the only thing visible was butt cheek) which again, is their preference. I can think of one "fashion" reason why someone would go commando in football pants.
If you are going to wear both a jockstrap and a girdle, wear the jock under the girdle. It gives better support that way, and the girdle smooths out the lumps and ridges of the jockstrap. In the good old days, most guys just wore a jockstrap, and it worked. Today many wear the girdle and not a jock, and it makes a lot of sense, and many would say it is more compfortable that way, with the cup inserted in the girdle. Thus, from the skin outward, you have the jock, the girdle, and the pants. It works for me.
It depends, I mean you can go without, but I usually wear underwear with them. If its something like plain spandex(nike pros for example) I've gone without and its fine but it depends on the spankies or briefs, and its up to you.
Underwear is not necessary under leotards, and when it peeks out, it looks unproffessional.
put it in the pocket of your jockstrap/ compression shorts and slip it on under everything. make sure you don't have on underwear under it or any other clothing because it will not be tight enough against your body.
That is called a girdle.
Strap
underwear
Some women wear underwear under tights and some do not. Tights are made to be worn without underwear, but many people do still wear them.
I prefer underwear, but you can wear either.
Some people do, but most people wear underwear.
For girls, only a bra. No underwear because it will show under you leotard. For boys, underwear thatis it under tights Leo and shorts
You wear your underwear.
Underwear...