An Illinois corporation owned a fleet of tank cars which were used to carry oil from the corporation's Oklahoma refinery to other States. The State of Oklahoma sought to impose a tax upon the entire fleet, although the cars were outside of Oklahoma at least twenty days each month. Held, Oklahoma had only the jurisdiction to tax the average number of tank cars habitually employed within the State. Johnson Oil Refining Co. v. Oklahoma, 290 U.S. 158, 54 Sup. Ct. 152 (1933).