The Improved Clinch knot is one of the most widely used fishing knots. It provides a good method of securing a fishing line to a hook, lure, or swivel. The "improved" version includes an extra tuck under the final turn. It is commonly used to fasten the leader to the fly. Because it is harder to tie in heavier lines, it is not recommended if you are using over 30lb test line.
The Improved Clinch knot is regarded as a fisherman's reliable standby. It is particularly suited for attaching a small diameter tippet to a heavy wire hook. The extra final tuck improves your chances of holding a strong fish.