… is from page 65 of the 2011 revised and enlarged edition of Thomas Sowell’s 2009 book Intellectuals and Society :
If, at a given time, three-quarters of the consumers prefer to buy the Acme brand of widget to any other brand, then Acme Inc. will be said to “control” three-quarters of the market, even though consumers control 100 percent of the market, since they can switch to another brand of widgets tomorrow if someone else comes up with a better widget, or stop buying widgets altogether if a new product come along that makes widgets obsolete.
DBx: When I was a young man, a large share of the market for devices used to create written documents was “controlled” by Smith-Corona – until it wasn’t.