Catnic Components had a patent for a steel lintel, used to provide structural support over a door or window opening in a brick wall.
The lintel is hollow, being made from sheet steel pressed into a rectangular or trapezoidal shape with a wind to anchor the device to the surrounding brickwork.
Hill & Smith created a virtually identical invention that had a bar that extended at an upwards slant, only 6 or 8 degrees from being completely vertical.
The Court of Appeal of England and Wales overturned the ruling as it held that the "vertical" requirement was an exact and essential element of the patent.
The Law Lords reversed the decision of the Court of Appeal, finding that there was an infringement, and affirmed the use of purposive construction in patent interpretation.