In addition to the states of Massachusetts and California, the butterfly knife was almost illegal in the state of Oregon, which forbids weapons which can be opened with the force of inertia or gravity alone. However, in a 1980's court case, it was successfully argued that movement of the thumb required to complete the opening manuever constitutes an element of skill, and moves the balisong out of the "Gravity Weapons" category.
Nonetheless, the butterfly knife has a bad reputation as a result of its being illegal in two states, and is commonly (mistakenly) believed to be illegal in all the states of the union. | 
