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I carry a Spyderco Tenacious. I wouldn't use it for self defense as a first option, or even a second option. A hand gun, a taser, or a baton would be more practical for defense, a knife just is not a practical weapon for self defense unless you are trained to use it for self defense. A hand gun, taser, or baton are fairly fool-proof, a knife requires some skill to use for self defense. In a self defense situation, you are trying to put an end to a threat, not merely mame or disfigure.
In any case, it is illegal in many states (I don't know if it's most or just a handful) to carry a knife for the intent of using it in self defense. There are also laws in many states against carrying "dangerous" knives. Utility knives such as swiss army, leatherman, boxcutters, buck knives, etc. are harmless, and most manufacturers make these kinds of knives under 4 and 3/4 inches (the legal limit of length for a folding knife in most states).
Any folding knife over that length in most states is highly illegal and impractical. Carrying any fixed blade knife of any length on your person (such as in a sheath or tucked in your belt) is also highly illegal in most states unless you are using it for outdoors purposes (hunting, fishing, camping, etc.)
The fact is, up until the 60's and 70's, there were a lot more ladies and gentlemen who carried a folding knife on their person, in a purse or coat pocket. It's because of exploitation films and James Dean movies that knives became associated with delinquency and thuggery.