A deer hair packer is a device used when tying flies with deer hair such as muddlers. Deer hair is "spun" many times onto the hook before finally being trimmed. The deer hair needs to be pushed together firmly before another batch is spun onto the hookshank to prevent gaps appearing. The thumb and index finger can be used to pack the deer hair together but a special tool called a deer hair packer is available. Just place it onto the hookshank apply slight pressure and push the deer hair together. Its a very simple tool but very effective.