A knife, whether fixed blade or an extending folding knife, concealed on someone's person capable of being used a "cutting weapon" is a felony (now a wobbler) in California. If the knife is in a sheath or similar which is not concealed is not a crime. The knife also must be ready to use. So a concealed folding knife w/o the blade folded out does not meet this criteria. Nor does a large fixed-blade knife in a sheath like the motorcycle gang guys carry.
For reference the California Penal Code is 21310 (previously 12020(a)(4)). Other states are obviously different.