The laws vary from state to state.
http://en.wikipedia.org/wiki/Knife_legislation#United_States_of_AmericaEach American state also has laws that govern the legality of carrying weapons, either concealed or openly, and these laws explicitly or implicitly cover various types of knives. Some states go beyond this, and criminalize mere possession of certain types of knives. Other states prohibit the possession and/or the concealed carrying of knives that feature blade styles or features sufficient to transform them into "dangerous weapons"[71][72] or "deadly weapons", i.e. knives either optimized for lethality against humans or designed for and readily capable of causing death or serious bodily injury.[71][73] These frequently include knives with specific blade styles with a historical connection to violence or assassination, including thrusting knives such as the dirk, poignard, and stiletto, and double-edged knives with crossguards designed for knife fighting such as the dagger.[71][74] Some states make the carrying or possession of any dangerous or deadly weapon with intent to unlawfully harm another a crime.[71]
In states like Alabama,[75] Mississippi,[76] New Mexico,[77][78] Texas,[71] and Virginia,[71] the carrying on one's person of large and lengthy fighting knives capable of causing grievous wounds such as the Bowie Knife[79] is prohibited by statute,[71][80] originally in the interest of controlling or eliminating the then-common practice of "dueling", a term which had degenerated from a rarely-used social custom into a generalized description for any knife or gun fight between two contestants.[74][81] In many jurisdictions a local tradition of using knives to settle differences or for self-defense[74][82] resulted in the enactment of statutes that restricted the size and length of the knife and particularly, the length of its blade.[74]