What are they?

I'm going to guess: two or more counts-to-10 within a boxing ring that has more than one set of ropes. :)

They are the KO's with several sizes that get progressively bigger.

But I like zylstra's answer better. :D

Cute. I'll assume that Ruben 718 is serious. It is possible, guys!

Electrical boxes often contain "knockouts", small pieces of plastic or metal that can be "knocked out" to provide for passage of wire. Some boxes have knockouts with several progressively larger rings and a solid circle in the middle. They share the same center -- hence the name "concentric". You knock out the size that you require.

Cute. I'll assume that Ruben 718 is serious. It is possible, guys!
I was serious too. :confused:

Now that I re-read your answer, I see that you were. Sorry 'bout that :cool:. I just looked at the "KO" and assumed you were talking about boxing also. I figured you were repeatedly knocking out the loser with bigger and bigger punches. I must be getting old...

I must be getting old...Or tired.
Or old and tired, like me. :rolleyes:

Thank you so much for the reply. I appreciated it.

One circle inside of another with the inside and outside lines spaced equal distances apart. Each ring must be completely removed, all the way around the circle. That is CONCENTRIC KNOCKOUTS. It identifies at the back of the panel box and left and right side view of the panel box. Use to enter straight connector or conduit pipe with thread and locknut.

Mr.ElectricBlue, the question was covered pretty clearly, and almost two years ago to boot. :o