The 1950 dispute Graver Tank v Linde Air Products established a principle that still governs patent analysis today: a product or process may infringe a patent even if it does not fall literally within the wording of the claims.

The 'function-way-result' test applies: if the substituted element performs the same function, in the same way, and achieves the same result as the patented element, there is equivalence and therefore infringement.

Why is this doctrine important? Because it enables patent proprietors to protect themselves against minor technical modifications designed to circumvent the claims without changing the essence of the invention.

In practice, equivalence is a technically and legally complex analysis, often carried out with the assistance of experts. When a competitor launches a product 'inspired' by your invention, the doctrine of equivalents is the first step in assessing an infringement action.