Market definition.
Advertisement  for Du Pont Cellophane, November 1948.
U. S. v. Du Pont
351 U.S. 377 (1956)
wDu Pont patents moisture proof  cellophane in 1927.
wDu Pont has 75% of cellophane market, but only 20% of the market for “flexible packaging.”
wCourt: cross-elasticity of demand between products.
wBut be careful: substitutes at competitive price not monopoly price.