§ 720 ILCS 5/33A-1(c)(2) is a source of confusion for knives with blades in excess of 3 inches such as Tony's Butterfly Knife. However, being "Armed with a dangerous weapon" is not a criminal offense in IL. It is merely an element of the offense Armed Violence. Except for during the commission of the felonies discussed in this section, being thusly armed is not criminal in IL, unless it is concealed. Therefore, the remaining criminal concern regarding this weapon lies in § 720 ILCS 5/24-1 (a)(2). This section makes it clear that carrying this type of knife is only unlawful when one has unlawful intentions. The most important ruling in IL regarding knife possession is People v. Gazelle which includes the clarification ". . .characterizing anything as a mere knife does not make it come under section 24-1(a)(1) of the Code; instead, a knife must either be a switchblade knife or a ballistic knife to come under section 24-1(a)(1) of the Code. . ."
In the City of Chicago, the weapons code is slightly stricter because many of the weapons that are legal to carry can not be concealed as per the Municipal Code 8-24-020 Carrying Dangerous Weapons. This does make carrying a knife with a blade longer than 2.5" a violation, but not a criminal offense. Furthermore, the carrying of dangerous weapons can not be threatening, menacing, or a breach of peace, regardless of the lawfulness of intent, as per 8-4-010 Disorderly Conduct. Also, care must be taken to adhere to the conduct expected by the Chicago Parks District.
Note also that the high courts in IL have issued other very relevant rulings regarding § 720 ILCS 5/24-1. This argument is particularly well laid out in MI People v. Malik: "The statute itself provides the cornerstone of our reasoning. It is very specific and lists the items which are illegal to possess." It seems that the Supreme Courts of every state interpret the law using the doctrine expressed in MI People v. Malik of "expressio unius est exclusio alterius", which means that "Where the legislature lists items in a statute, it is the general rule that express mention of one thing implies the exclusion of other similar things." In Illinois, see People v. Fink (Appellate Court), People v. Fink (Supreme Court)and People v. Tate. The later uses this maxim and the fact that nunchaku are not actually bludgeons. In a related case, LexisNexis explains the legislative intent of this ruling which is that nunchaku were not per se dangerous or deadly weapons for purposes of possession of unlawful weapons statutes as follows: ...the court stated that "numchucks," utilized in the legitimate sport of karate, are not "bludgeons" within the meaning of the unlawful-use-of-weapons [***3] statute. Therefore, their possession was not prohibited under section 24 -- 1(a)(1) of the Criminal Code of 1961 (Ill. Rev. Stat. 1977, [*384] ch. 38, par. 24 -- 1(a)(1)).
Be advised that regulations regarding weapons usage vary by jurisdiction although a serious martial artist is generally allowed to practice his martial arts curriculum wherever he is as long as it is not a nuisance. While visiting Florida, I had occasion to research their well written laws. Here are some excerpts from the Florida Statute Title XLVI (CRIMES) - Chapter 790 (WEAPONS AND FIREARMS).