The leading or cutting edge of the above propeller is the lower edge. The higher edge above is the trailing edge. This edge sometimes has a hook or a rolled edge added to it which is called a "cup".

By cupping a propeller it will help to improve efficiency and provide a better grip of the water. I liken the cup to the difference of swimming with your hands perfectly flat to that of swimming with cupped hands. The cup holds the water on the propeller blade and reduces slippage.

Slippage is the loss of the propellers ability to cut at its given pitch. See Pitch

Cup can be Single cup or Double cup.

This lip generally helps a prop hold water. It also adds the equivalent of about 1/2-inch to one-inch of pitch.