It is almost 60 years since Heinz Schild devised a method that allowed measurement of a genuine physical quantity, the equilibrium constant for binding of a competitive antagonist. The clever bit was that the measurements could be made using responses from tissues despite the fact that little or nothing was known about how the agonist worked. Since then, attempts have been made to generalise the Schild equation, but they are all based on false premises. It turns out that generalisation is usually not needed. His original simple result is still valid in cases where several agonist molecules must be bound to produce a response, even if the agonist binding sites interact or are not identical.