20mm Neodymium Ring Magnets - 

The N822 is a neodymium (rare earth) Ring Magnet with a concentric hole inside it. A Neodymium countersunk magnet is a Neodymium ring magnet with one side appearing as a standard ring but the other side has an angled countersink to take a screw head, the angle usually being (but not always) 90°. A Neodymium counterbore ring magnet is a ring magnet with a cylindrical flat-bottomed hole on one side that enlarges the main central hole to take a cap head screw. All these Neodymium ring magnet variations are nearly always axially magnetised - the magnet is magnetised along its central axis. So one flat end face is a North pole and the other flat end face is a South pole. For pure rings, turning over the ring changes the polarity. But for the NdFeB countersunk rings and the NdFeB counterbore rings the countersunk or counterbore side will be a North or a South and the other end (which will visually look like a pure ring) will have the opposite polarity).