Hareline Flat Lead Wire is a fly tying material used to add weight to fly patterns, particularly nymphs and streamers. The wire is made from lead, which is a dense, heavy metal that helps the fly sink quickly in the water column. The flat shape of the wire provides a smooth profile and helps prevent the wire from rolling or twisting around the hook shank.

It's important to note that lead is a toxic material, and fly tyers should take appropriate safety precautions when handling and disposing of Hareline Flat Lead Wire, including using gloves and avoiding ingesting or inhaling the material.