A neck knife is a small fixed-blade knife which is carried by means of a "pendant rig", in which the knife is suspended from around one's neck, handle down, by either a length of paracord or, more commonly, a breakaway beaded or ball chain such as those utilized for military dog tags.
The knife stays in place by means of a form-fitting synthetic sheath, which holds it securely in place until yanked sharply. Some manufacturers prefer a looser fitting sheath augmented with magnets.
Neck knives are usually single-edged, with blade lengths typically under four inches – and frequently less than three inches. They are primarily intended for utilitarian use, although non-utilitarian versions (i.e., daggers and T-handled push daggers) also exist.
Neck knives are most frequently worn around the neck, but may be suspended from under the arm as well. They are sometimes worn under one's shirt for concealment, although this makes a quick draw nearly impossible, and even simple retrieval for utility purposes awkward. They are more frequently worn outside of a shirt. Not only does this make drawing the knife far easier, but it also avoids legal issues in jurisdictions where concealed knives are regulated.
Taken fron Wikipedia