Wisconsin Knife Laws
Basically, in WI you can carry whatever the hell you want, as long as it's done openly (which is something of a trick, since we've got 7-9 months of winter here).
However, a 2.75" knife, if concealed, can be counted as a "dangerous weapon", and has been by courts in the past few years (they don't otherwise say what constitutes that. Some cities have defined it themselves)
They also don't quite specify exactly what counts as concealed. They have a definition, but there are grey areas, like a pocket knife with a pocket clip and that has part of it stick out of the pocket and be visible.
Basically, to be entirely safe carrying a knife, the most basic and possibly most useful of all of mankind's myriad tools, you need a carry permit.