Neck Knives are carried on a lanyard or cord around your neck, hence the name, neck knives. They are small knives that are easily accessible since they are right on your person.
The sheaths for each neck knife have good retention qualities, so your blade won't slip out until you want it to. Worn under your shirt, you'll have a concealed weapon at your disposal.
These types of knives are good for surprise counterattacks where your assailant doesn't think you have a weapon to defend yourself with. You can very quickly show him wrong with quick stabbing motions to inflict maximum damage.
Wearing a neck knife around your neck conceals your defensive capabilities while providing you with a proven self-defense product you can depend upon.