Courts may use the doctrine of equivalents to find an accused infringer liable for patent infringement, even if the infringing method or device does not literally infringe the patent.
For example, if one element of a patent claim directed to an improved mousetrap required "a metal screw," and the infringing device satisfied all of the other elements except "a metal screw", instead using "a wooden screw", a court nonetheless may find liability here.
In the U.S., the test was announced in the landmark case Warner-Jenkinson Co. vs. Hilton Davis Chem. Co:
"whether the difference between the feature in the accused device and the limitation literally recited in the patent claim is "insubstantial."
When is the difference considered "insubstantial"?
The difference is insubstantial when the accused device:
1. Performs substantially the same function
2. in substantially the same way
3. to yield substantially the same result.
For example, if one element of a patent claim directed to an improved mousetrap required "a metal screw," and the infringing device satisfied all of the other elements except "a metal screw", instead using "a wooden screw", a court nonetheless may find liability here.
In the U.S., the test was announced in the landmark case Warner-Jenkinson Co. vs. Hilton Davis Chem. Co:
"whether the difference between the feature in the accused device and the limitation literally recited in the patent claim is "insubstantial."
When is the difference considered "insubstantial"?
The difference is insubstantial when the accused device:
1. Performs substantially the same function
2. in substantially the same way
3. to yield substantially the same result.
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