Badger hair shaving brush.

A shaving brush should be filled with real badger hair (and not substitute badger hair or bristles), because:
-Badger hair is very fine and very elastic, even when wet.
-It is is not porous and has only a fraction of the diameter of pig bristles (from which imitation badger hair is made for shaving brushes).
-With badger hair (and a good shaving soap), you can work up a very fine and creamy lather, which is what you need to make shaving gentle on your skin.