Background
International Game Technology (IGT) held patents covering software-controlled slot machine bonus game systems — specifically, systems that allow players to participate in a “secondary game” triggered by certain outcomes in the primary slot machine spin, with the secondary game outcome affecting the player’s award. WMS Gaming manufactured competing slot machines with similar bonus features and IGT brought infringement claims. WMS moved to invalidate the patents as indefinite, arguing that the functional claim language describing the game controller’s bonus determination functions lacked adequate structural support in the specifications.
The district court construed the claims and found infringement. WMS appealed on both claim construction and validity grounds, specifically targeting the means-plus-function analysis and the adequacy of the specification’s algorithm disclosure.
The Court’s Holding
The Federal Circuit vacated and remanded for reconsideration of the claim construction. The court applied the established rule for means-plus-function claims under § 112(f): when a claim element is expressed as a means for performing a specified function, the claim is limited to the corresponding structure described in the specification and its equivalents. For computer-implemented means-plus-function claims, the corresponding structure is the specific algorithm described in the specification — not merely the general-purpose computer on which it runs.
The court found that the district court had not properly identified and applied the specific algorithms described in the specification as the corresponding structure for the software-implemented bonus game functions. Without identifying the precise algorithmic structure corresponding to each functional claim element, the court could not properly determine infringement (whether the accused system performed the same function by substantially the same means), equivalents (structural equivalents), or validity (whether the specification disclosed adequate structure). The case was remanded for claim construction to identify those algorithms.
Key Takeaways
- For computer-implemented means-plus-function claims, the “structure” required by § 112(f) is the specific algorithm the computer is programmed to execute — not a generic reference to a computer or software program.
- Infringement of a means-plus-function claim requires that the accused product perform the identical function by substantially the same means (the corresponding algorithm or its structural equivalent).
- Claim construction must identify the specific algorithm in the specification corresponding to each means-plus-function limitation before infringement or validity can be properly analyzed.
- Patent drafters should explicitly describe the algorithms implementing each claimed function — in flowcharts, pseudocode, or step-by-step descriptions — to provide adequate corresponding structure under § 112(f) and to maximize claim enforceability.
Why It Matters
WMS Gaming v. IGT contributed to a line of Federal Circuit decisions in the mid-2000s that rigorously applied the § 112(f) algorithm-disclosure requirement to software patents — reinforcing that software-implemented means-plus-function claims cannot rely on generic computer language as their corresponding structure. This approach — developed in parallel with Aristocrat Technologies v. IGT (2008) and reinforced in subsequent cases — established a demanding disclosure standard for software patent claims that remains a critical consideration in patent prosecution and litigation.
For the gaming machine industry — in which sophisticated software systems control virtually all aspects of modern slot machine play — the ruling meant that patent holders needed to carefully document the specific algorithms implementing each claimed function. Broader claims using vague functional language without algorithmic support faced invalidity exposure, while precisely drafted claims supported by algorithmic disclosure maintained their enforceability. The same principles apply across all technology sectors that use computer-implemented systems.