Midi rings sit in the middle of a finger, right above the lower knuckle. This jewelry is typically a few sizes smaller than your regular ring size so that they fit snugly in the center of a finger. They are also sometimes called "knuckle rings" or "above knuckle rings."
They come in many materials and styles, from whisper thin minimal midi rings that seamlessly blend with your other jewelry, to hammered rings, small signet rings, and other designs.