Dual-purpose fox snares can be used in either self-locking or free-running positions, depending on the way in which the metal wire is threaded.
A dual-purpose snare consists of a small metal bracket with three holes. One end of the wire is anchored through the first hole of the bracket, and a noose is then formed with the other end of the wire passing through one of the other two holes in the bracket; this end is then attached to the stake, fence post or whatever. If the tail-end of the wire is passed through the middle hole of the bracket, it forms a free-running snare. If however it passes through the top hole it forms a self-locking snare.
Although on the face of it free-running snares are more humane because they will slacken off, in practice it is easy for kinks to form in the wire of these snares, turning them into self-lockers.