The hopper-dropper is a large dry hopper [or BLT,or Club Sandwich, or Chero Ant] with an 18-24" lighter tippet tied off the hook bend with a hare's nymph, pt, or something similiar dropped off the back. The fish may hit the hopper - or the dropper nymmph. If they hit the nymph the hopper acts as a "bobber". It's quite a productive system at times.

Donald