Here is just a riddle, which I ask you to solve.
What do the following stories have in common?
1. During the Middle Ages, it became impossible to carry on commercial trade on the Rhine River because of the toll booths built by all sorts of barons.
2. Augustin Cournot explained in 1838 that the price of brass would fall if a monopoly zinc producer and a monopoly copper producer could merge.
3. In the second decade of the twentieth century, it was almost impossible to build an airplane in the United States.
4. Filmmakers must often make significant changes in their work because of the costs or complications of rights clearances.
Use you brain (and/or your favorite search engine).