Geez, guys, let's not all fall over ourselves in our haste to make goofy assumptions about laws we don't understand.
Did ANYBODY who posted "that's illegal, you doer of illegal things!" actually check the laws in SWEDEN, where Eurohacker lives? Or did you post your understanding of your local and state laws as if they applied to the whole U.S., much less the entire globe?
In Illinois, for instance, you can legally carry any size knife you want, concealed any way you want, as long as you're not carrying it "with intent to illegally harm another."
In California, you can't conceal a fixed blade, but if an inch or so is visible, there's no length limit. Jim March's Outsider, back in the day, was designed as long knife for "outside" fighting and he had the sheath fitted so that an inch of the pommel was visible below his jacket. Perfectly legal.
As for "Rules of Thumb," my personal rule of thumb is that if somebody tells you there's a "Rule of Thumb" that has to do with how many fingers long your knife is, it was probably made up by some cop or security guard. 9 times out of ten, if you check the law, it's NOT in there. I've heard so many length limits for Illinois you wouldn't believe it--three inches, four inches, three and a half, four fingers, three fingers, six inches, etc. etc. etc.
NONE of those are in the law.