Coupling and Cohesion: The Two Design Principles That Define Maintainable Code

Cohesion and coupling are two foundational software design concepts that determine how well a codebase can be understood, tested, and changed over time. Cohesion refers to how closely related the responsibilities within a single module are, while coupling measures how dependent one module is on another. Well-designed systems aim for high cohesion within modules and low coupling between them, reducing the risk that a change in one area breaks unrelated functionality elsewhere. As systems grow, poor design choices — such as bundling authentication, billing, and reporting into one module — make even small updates costly and error-prone. Applying these principles helps developers clearly separate concerns, making it easier to locate, modify, and scale individual parts of a system independently.