720 ILCS 5/24-1 (2013) Unlawful Use of Weapons
(a) A person commits the offense of unlawful use of weapons when he knowingly:
(1) Sells, manufactures, purchases, possesses or carries any [
] metal knuckles or other knuckle weapon regardless of its composition, throwing star, or any knife, commonly referred to as a switchblade knife, which has a blade that opens automatically by hand pressure applied to a button, spring or other device in the handle of the knife, or a ballistic knife, which is a device that propels a knifelike blade as a projectile by means of a coil spring, elastic material or compressed gas; or
(2) Carries or possesses with intent to use the same unlawfully against another, a dagger, dirk, billy, dangerous knife, razor, stiletto, broken bottle or other piece of glass, stun gun or taser or any other dangerous or deadly weapon or instrument of like character; or
(9) Carries or possesses in a vehicle or on or about his person any pistol, revolver, stun gun or taser or firearm or ballistic knife, when he is hooded, robed or masked in such manner as to conceal his identity;
(b) Sentence. A person convicted of a violation of subsection 24-1(a)(1) through (5) [
] commits a Class A misdemeanor. A person convicted of a violation of subsection [
] 24-1(a)(9) commits a Class 4 felony; [
] A person convicted of a second or subsequent violation of subsection [
] 24-1(a)(9) commits a Class 3 felony. The possession of each weapon in violation of this Section constitutes a single and separate violation.
720 ILCS 5/33A-1 (2013) Legislative intent and definitions
(a) Legislative findings. The legislature finds and declares the following:
(1) The use of a dangerous weapon in the commission of a felony offense poses a much greater threat to the public health, safety, and general welfare, than when a weapon is not used in the commission of the offense.
[
]
(1) Armed with a dangerous weapon. A person is considered armed with a dangerous weapon for purposes of this Article, when he or she carries on or about his or her person or is otherwise armed with a Category I, Category II, or Category III weapon.
(2) A Category I weapon is a handgun, sawed-off shotgun, sawed-off rifle, any other firearm small enough to be concealed upon the person, semiautomatic firearm, or machine gun. A Category II weapon is any other rifle, shotgun, spring gun, other firearm, stun gun or taser as defined in paragraph (a) of Section 24-1 of this Code [720 ILCS 5/24-1], knife with a blade of at least 3 inches in length, dagger, dirk, switchblade knife, stiletto, axe, hatchet, or other deadly or dangerous weapon or instrument of like character.
(3) A Category III weapon is a bludgeon, black-jack, slungshot, sand-bag, sand-club, metal knuckles, billy, or other dangerous weapon of like character.
I read it as such: In Illinois (unless in a public school or unless other municipality laws intercede) there are no limits on blade length. Intent to cause someone else harm with a knife is considered an Unlawful Use of a Weapon. However, intent has to be proven. Just carrying a knife is not enough to prove intent. During the commission of a crime, carrying a blade over 3 inches in length is considered "Armed with a Dangerous Weapon" which carries a separate charge.