Taken from the Chicago Municpal Code is the following. I read it to prohit possession of a knife with a 2.5" blade REGARDLESS OF INTENT. The legal length is shortend to 2" if the person is under 18 years of age.
8-24-020 Carrying dangerous weapons.
No person shall sell, offer for sale, keep, possess, loan or give to any person any knife, the blade of which is released by a spring mechanism, including knives known as "switch-blades," any blackjack, slingshot, sandclub, sandbag, metal knuckles or bludgeon. No person shall sell, offer for sale, loan or give to any person 18 years of age or under any type or kind of knife, any blade of which is two inches in length or longer.
No person shall sell, manufacture, purchase, possess or carry any weapon from which eight or more shots or bullets may be discharged by a single function of the firing device.
No person shall carry or possess any knife, the blade of which is released by a spring mechanism, including knives known as "switch-blades," any blackjack, slingshot, sandclub, sandbag, metal knuckles or bludgeon. No person 18 years of age or under shall carry or possess any knife, the blade of which is two inches in length or longer.
No person shall carry or possess with intent to use same unlawfully against another a dagger, dirk, billy, dangerous knife, razor, stiletto or other dangerous or deadly weapon.
No person shall carry concealed on or about his person a pistol, revolver, derringer or other firearm or dagger, dirk, stiletto, bowie knife, commando knife, any blade of which is released by a spring mechanism, including knives known as "switch-blades" or any other type or kind of knife, any blade of which is more than two and one-half inches in length, ordinary razor or other dangerous weapon except that no person 18 years of age or under shall carry concealed on or about his person, any knife, the blade of which is two inches in length or longer. This provision shall not apply, however, to the following officers while engaged in the discharge of their official duties: sheriffs, coroners, constables, policemen or other duly constituted police officers and wardens, superintendents and keepers of prisons, penitentiaries, jails and other institutions for the detention of persons accused or convicted of crime; nor to the following employees or agents while engaged in the discharge of the duties of their employment: conductors, baggagemen, messengers, drivers, watchmen, special agents and policemen employed by railroads or express companies; nor to persons lawfully summoned by an officer to assist in making arrests or preserving the peace, while so engaged in assisting such officer.
Any person violating the provisions of this section shall be fined $200.00 for each offense, or shall be punished by imprisonment for a period of six months, or by both such fine and imprisonment.
In addition to all other penalties, weapons used in violation of this section shall be forfeited to, and confiscated by, the city.