In the world of software litigation, very few disputes are as deceptively complex as determining whether one program is “substantially similar” to another. Unlike plagiarism in prose, where copied sentences often betray the act, software is written in ways that make verbatim copying unnecessary. A developer may use different variable names, restructure functions, or change comments, while still replicating the heart of another’s work. Courts have developed the concept of substantial similarity to address these situations, looking beyond literal line-by-line matches to evaluate whether the expression of ideas in code has been misappropriated.

This article explores what substantial similarity means in practice, why it matters in software copyright disputes, and how judges and juries rely on technical testimony to make sense of the issue.

Why “Substantial Similarity” Exists in Copyright Law

Copyright protects the expression of ideas, not the ideas themselves. This is true in literature, music, and film, but it’s especially complicated in software because code straddles two worlds. On one hand, it is expressive, reflecting design choices, stylistic conventions, and problem-solving creativity. On the other, it is functional—computers don’t care about style, only about instructions that compile and execute.

If copyright law only prohibited verbatim copying of code, infringement would be trivially easy to avoid. A competitor could rewrite functions with different syntax but identical logic, and the original author’s work would be effectively unprotected. To close this gap, courts developed the substantial similarity test, asking whether the alleged copy appropriates the “total concept and feel” of the work, even if it looks different on the surface.

Landmark Cases Defining Substantial Similarity in Software

Several cases illustrate how courts wrestle with this concept:

• Apple Computer, Inc. v. Microsoft Corp. (1994): This case examined whether Microsoft’s Windows graphical user interface unlawfully copied Apple’s Macintosh interface. The court recognized that not all similarities matter—many elements were dictated by function or industry standards. Only protectable, creative aspects were relevant.
• Computer Associates Int’l v. Altai (1992): Known for creating the “abstraction-filtration-comparison” test, this case established a structured approach. Judges first strip away unprotectable elements like ideas, efficiency-driven decisions, and industry-standard conventions, then compare what remains for similarity.
• Oracle America, Inc. v. Google Inc. (2016): This high-profile case asked whether Google’s use of Java APIs in Android was infringing. The courts considered whether APIs, as organizational structures, are expressive enough to merit copyright protection.

For broader reading, Cornell Law School’s Legal Information Institute offers a solid primer on how copyright applies across different media, including software. Stanford Law’s overview of software copyright cases gives a helpful historical perspective

Beyond Literal Copying: What Courts Examine

To unpack substantial similarity, it helps to consider the categories of code features that often come under scrutiny:

1. Structure, Sequence, and Organization (SSO)
   A program’s architecture—the way modules, classes, and functions are divided and arranged—can reflect significant creative judgment. Two systems with identical SSO may suggest copying, even if their actual code is rewritten.
2. Non-literal Elements
   Courts often consider whether flowcharts, pseudocode, or design documents reflect unique expression. For example, two pieces of software that share the same unusual data structure design may be more than coincidence.
3. Look and Feel
   User interfaces, though partially functional, can also display originality. The choice of layout, interactive flow, and even the combination of menus and icons can tip the scale toward similarity.
4. Algorithms and Implementation Choices
   While common algorithms like quicksort are public domain, the particular way a developer implements them may involve creative decisions. If a unique approach is mirrored, courts may view this as infringing.

The challenge is distinguishing what is necessary for functionality from what reflects individual creativity. That’s where a software expert witness often plays a pivotal role.

The Role of the Software Expert Witness

Because software disputes often involve tens of thousands of lines of code, judges and juries need guidance. A software expert witness can clarify technical nuances, explaining why two codebases are similar—or why they only appear to be.

For example, if two systems both use a hash table for data retrieval, that may reflect a standard, efficient solution rather than copying. But if the same unusual sequence of function calls or error-handling routines appears in both, the similarity may be telling. The expert’s job is to map these technical findings into the legal framework of substantial similarity.

Courts have increasingly emphasized the importance of this testimony. Without an expert, juries may either underestimate similarity (because code looks different on the surface) or overestimate it (because all code looks “technical” and therefore unique to a layperson). A strong analysis helps ensure fair outcomes.

Practical Examples of Substantial Similarity

To make this less abstract, consider a few hypotheticals:

• Literal Copying Avoided, Structure Retained
  Developer B rewrites Developer A’s e-commerce platform in a new programming language. None of the variable names or syntax are the same, but the entire module structure—shopping cart, checkout flow, and payment processing—is organized identically. A court may see this as substantial similarity.
• Shared Algorithms vs. Shared Expression
  Two video game developers use pathfinding for character movement. Both implement the A* algorithm, but one developer writes a custom heuristic that is mirrored by the other. The algorithm itself is unprotectable, but the unique heuristic may count as expression.
• User Interface Copying
  If two productivity apps feature nearly identical menu layouts, toolbars, and workflow sequences, a judge may conclude that protectable elements of design were copied—even though the underlying code differs.

These examples illustrate how substantial similarity reaches beyond the obvious.

Balancing Protection and Innovation

There’s an inherent tension in these cases. Too much protection risks stifling innovation by preventing developers from using common techniques. Too little protection leaves original creators vulnerable to misappropriation. The abstraction-filtration-comparison test was designed to strike this balance, but applying it remains case-specific.

For deeper policy discussion, the Electronic Frontier Foundation has argued for careful limits on software copyright to avoid chilling innovation, while academic perspectives from the Harvard Berkman Klein Center highlight how difficult it is to separate expression from function.

Why Substantial Similarity Matters for Businesses

For technology companies, the stakes are enormous. A finding of infringement can lead to injunctions, damages, and reputational harm. On the other hand, defending against an infringement claim requires substantial investment in legal and technical resources.

This is why many businesses rely on a software expert witness not only in litigation but also in risk assessment and due diligence. Before acquiring a company, releasing a product, or responding to a competitor’s claims, expert analysis can help clarify whether similarities are legally risky or simply the result of convergent engineering.

Conclusion

Determining whether code is substantially similar is never a purely mechanical exercise. Courts must parse legal precedent, strip away unprotectable elements, and listen to detailed technical testimony before deciding whether a program unlawfully copies another. Literal matches are only the starting point; the deeper questions involve architecture, algorithms, design choices, and originality.

At Sidespin Group (https://sidespingroup.com), we help businesses navigate these complexities. Our team provides AI strategies for companies looking to innovate responsibly and also offers software expert witness services for litigation matters, ensuring that both courts and clients receive clear, grounded, and actionable insight.