After following rec.knives for a while, it became obvious that a quick summary of knife laws would be handy. Here's what little I've been able to glean from my reading.

My goal is to find the clear portion of what is flat-out legal and easily summarized. I realize most states confuse the issue unmercifully; I'll let others figure out how to push the limits.

WARNING: This document is created from hearsay and whatever laws I could find. For legal advice, ask a lawyer. I could be lying. I only provide this to try to slightly improve the general lack of information on this subject. YOU are responsible for your own actions. If you don't know exactly what the laws are for any state or locale you are in, GO FIND OUT. (http://www.ncsl.org/public/sitesleg.htm is a good start, containing pointers to all state legislative sites.) I haven't updated this for a while, so consider it a cursory guide.

Explaination

• Summary:

Basically, I want to answer the question "I'm flying to state X tomorrow, so which knife can I take?"
• Max length:

Size limit, measured the most unpleasant way possible. Some may permit longer in certain cases, but I won't suggest anything longer if it's in a gray area.
• Specifically illegal:

Specific styles which are explicitly forbidden.
Switchblade = Push a button/lever, it does the rest.
Gravity knives = Opened by gravity or centrifigual force. Butterfly knives (balisongs) included.
• Relevant laws:

A quick pointer to roughly where the relevant laws are (ex. Penal Code 642)
• Quirks:

Dark humor points (ex: a 1" lockback in a pocket is illegal, but a 15" Bowie swinging free is ok)
• Local restrictions:

City/county restrictions.

US Knife Law Summary

• Ballistic knives (automatic blade throwers) are prohibited.
• Switchblades may not be transported/mailed across state lines unless the recipient intends to use it for military use.
• Possessing a knife in a federal facility is prohibited unless it is a pocket knife with blade under 2.5".

State Knife Law Summaries

• Summary: Generally clear. Illegal to carry "deadly weapon, (except a pocket knife) concealed."
• Max length: None apparent.
• Relevant laws: ARS book

• Summary: Anything capable of ready use as a stabbing weapon is illegal. All concealed knives are a felony (except non locking folders).
• Max length: chaotic
• Specifically illegal: Switchblades, gravity knives, exotic conceal methods (pen, lipstick, whatever)
• Relevant laws: PC 653K, PC 12020(24), California Laws, Commentary (apparently removed - someone know where?)
• Note that interpretations by cops and judges are wildly varied. Police have actually told law-abiding citizens to break the concealment laws, and a judge has deemed a blunt-point knife (Spyderco Mariner) a stabbing weapon.
• Quirks: Pens may be illegal (potential stabbing weapon).
• Local restrictions:
• Los Angeles
• Three inch limit for open carry without a good explanation.

• Summary: Sounds normal.
• Max length: Cutting edge under 4 inches
• Switchblades: Are legal to carry with a Dangerous Weapons Permit (DWP) only. Good luck trying to find a police station that has an application.
• Gravity knives: Same
• Relevant laws:
• Local restrictions: Some police chiefs don't want to give out DWP's

• Summary: A "common pocketknife" is OK. All "concealed weapons" (knives included) require a license.
• Max length: None apparent.
• Relevant laws: Chapter 790

• Local restrictions:
• Atlanta: A blade over 3" that LOCKS is illegal.

• Summary: Generally OK.
• Max length: None apparent.
• Specifically illegal: automatics & throwing stars.
• Relevant laws: Statutes mentioning "knife"

• Summary: OK for "normal" knives. Don't try to conceal any bowie, dirk, switchblade or butcher knife.
• Max length: None apparent.
• Specifically illegal: none apparent.
• Relevant laws: 97-37-*
• Quirks: Threatening actions with a knife in the presence of less than three people may be acceptable.

• Summary: any folder 4" or less is OK.
• Max length: 4"
• Relevant laws: State Laws (search for "knife")

• Summary: "Penknives" are OK. Anything else is borderline illegal and may require a concealed weapon permit.
• Max length: None apparent.
• Relevant laws: MD Statues Crimes and Punishments § 36, § 36A-O
• Quirks: You may carry a "weapon as a reasonable precaution against apprehended danger", but it's up to a tribunal to decide the reasonableness/appropriateness of posession.
• Local restrictions:
• Cecil, Anne Arundel, Talbot, Harford, Caroline, Prince George's, Montgomery, St. Mary's, Washington, Worcester, Kent, and Baltimore Counties have special prohibitions regarding children under 18 carrying knives. See § 36(a)(3).

• Summary: Nothing over 3.5"
• Max length: 3.5"
• A longer blade may be legal, but it's subject post-fact to a judge's decision.
• Relevant laws: Statutes mentioning "knife"
• Statutes may refer only to concealed knives.
• Quirks:
• A "knife" is defined as having a blade over 3.5". A pocketknife under 3.5" is not a knife.
• Local restrictions:
• Linconln
• Switchblades are illegal.

• Summary: General folders OK. Single-edged fixed blades may be.
• Max length: Under 18 may not possess knife with 5" or longer blade, or 10" or longer overall. No other apparent limitation.
• Specifically illegal: gravity knife, switchblade knife, dagger, dirk, stiletto, or ballistic knife "without any explainable lawful purpose" (i.e. an ill-defined exemption). Manufacturers and sellers are not exempt.
• Relevant laws: 2C:39-3.e 2C:39-9.d 2C:39-9.1 (Statutes, search for "knife" or "knives")

• Summary: If it looks like a weapon, it's illegal.
• Max length: 6" (?)
• Specifically illegal: Switchblades and gravity knives unless hunting or fishing with permit
• Relevant laws: Penal law
• Local restrictions:
• New York City
• Must be under 4"

• Summary: Generally OK.
• Max length: None apparent.
• Specifically illegal: switchblades, belt-buckle knives
• Relevant laws: NRS 202

• Specifically illegal: switchblade, springblade knife, gravity (butterfly) knife, or similar weapon;
• Relevant laws: Search Statutes for "knife"

• Summary: 3" or less OK. Don't posess anything "designed to cut and stab another".
• Max length: 3" measured from where the handle ends, not where the sharpened edge begins.
• Specifically illegal:
• Posession of a dagger, dirk, stiletto, sword-in-cane, bowie knife, or other similar weapon designed to cut and stab another.
• Concealed carry upon one's person of the above-mentioned instruments or weapons, or any razor, or knife of any description having a blade of more than 3".
• Relevant laws: Title 11 Criminal Offenses § 11-47-42
• Quirks: Children under 18 may purchase the above weapons with written parental permission.

• Summary: Folders under 4" are OK.
• Max length: 4"
• Specifically illegal: Switchblades, gravity knives (probably)
• Quirks: Fixed blades are probably a no-no.

• Summary: Folders under 5.5" OK.
• Max length: 5.5"
• Specifically illegal: Switchblade, throwing knives, daggers (in general), bowie knives, swords and spears.
• Relevant laws: Penal Code 46
• Quirks: The one state people associate with Bowie knives explicitly forbids them.

• Summary: Don't conceal a dirk or bowie knife. Don't take a dangerous weapon (esp. bowie knife or dagger) to church.
• Max length: None apparent.
• Specifically illegal: Switchblades.
• Relevant laws: 18.2-308, search statutes for knife or knives.
• Quirks: 3.1-370: your knife must be cleaned daily.

• Summary: Anything over 3" is in a gray area.
• Max length: 3"
• Specifically illegal: switchblade, springblade knife, gravity (butterfly) knife, concealed dagger/dirk
• Relevant laws: Statutes mentioning "knife"

• Relevant laws: 134.71 (1)(a)9, 134.71 (1)(g)1, 134.71 (1)(h)1  (relating to pawn brokers), 941.24 (switchblades)

Canada

• Summary: Careful; the country is getting antsy about weapons.
• Specifically illegal: a knife that has a blade that opens automatically by gravity or centrifugal force or by hand pressure applied to a button, spring or other device in or attached to the handle of the knife.
• Relevant laws: Annual Statutes Of Canada, 1995 Chapter 39 (Bill C-68)

Airlines