The Continua of Excludability and Rivalry

The classic definition of a “public good” is that it is both “non-excludable” and “non-rival.”  Textbooks normally treat these traits as binary, delivering this 2 x 2 typology:

Yet in the real world, both excludability and rivalry lie on a continuum.  Almost nothing is 0% excludable.  If you spend enough effort, you can prevent non-payers from enjoying your product.  At the same time, almost every good requires some effort to exclude non-payers.  Hence, nothing is 100% excludable either.

Think ...