Actually they are called guards, the knife blades rivet to a bar and travel back and forth between the guards to cut hay. The individual knife blades are called sections. If you run through a gopher mound or a mouse or bird nest and it hangs on the guard, then you got to back up, lift the bar, get off and clean it off, get on lower it down ahead of your cut unless you have a power takeoff on a tractor, horse pulled the sections were driven by the wheel, so to keep if from plugging you had to have them going real good before you hit the hay you want to cut. Trust me, I've been there and done that. The rotating mowers they use now don't plug, you can run through cut hay or most anything but a wad of wire, and just keep going. Those came out just at the end of my farming career.