A hackle is a tool used in fly tying to help shape and organize the fibers of feathers, fur, or other materials that make up the fly. It consists of a long, thin length of wire or other material attached to a handle, and has rows of fine, metal teeth running along its length. Hackles are used to strip down feathers and fibers and to create a uniform, streamlined appearance for the fly. They can also be used to create various patterns and textures in the fly, depending on the shape and angle of the teeth.