Code is read far more often than it is written, and most of those readings are performed by someone other than the original author, frequently long after the context of the work has faded. Documentation is the mechanism by which a codebase remains maintainable across that gap of time and authorship. This article presents a practical approach to documenting a codebase so that a new maintainer can understand it, change it safely, and operate it, organized around the principle that different readers need different layers of documentation.
Document in Layers
A maintainable codebase is documented at several altitudes, each serving a distinct question. The highest layer explains the system: what it does, why it exists, and how its major parts fit together. The middle layer explains a module or component: its responsibility, its interface, and its dependencies. The lowest layer explains the code itself: why a particular implementation is the way it is.
These layers serve different readers arriving with different questions. A new contributor orienting themselves needs the system overview before any individual file makes sense. A developer fixing a specific bug needs the module documentation for the area they are touching. A maintainer modifying a tricky function needs the inline rationale for why it was written that way. Documenting only at one altitude — typically the lowest, in scattered comments — leaves the other readers unserved.
Write a README That Orients
The README is the entry point and deserves disproportionate care because it is the first and sometimes only document a reader consults. An effective README answers a predictable set of questions: what this project is and the problem it solves, how to set it up and run it locally, how the codebase is organized at a high level, and where to go for more detail. Setup instructions in particular should be complete and tested, because a reader who cannot run the project will not contribute to it.
The README should describe the shape of the codebase — the major directories and their responsibilities — so that a reader can locate the area relevant to their task without reading every file. It need not be exhaustive; its job is to orient, then point onward to deeper documentation where it exists.
Explain the Why, Not the What
Inline comments are most valuable when they explain why code exists, not what it does. Well-named functions and variables already convey what the code does; a comment that restates this adds noise and, worse, becomes a second source of truth that can drift out of agreement with the code. The information that code cannot express is intent: why this approach was chosen over an obvious alternative, what constraint or edge case the logic accommodates, or why a seemingly redundant step is in fact necessary.
A comment that records a non-obvious decision saves a future maintainer from re-deriving the reasoning or, worse, removing the apparent redundancy and reintroducing the bug it prevented. Public interfaces benefit from documentation comments describing parameters, return values, and error conditions, so that a caller can use the function correctly without reading its implementation.
Keep It Clear and Current
Documentation is subject to the same standards as any technical writing: it should address its reader plainly, use short sentences and consistent terminology, and prefer the active voice and present tense, as established documentation style guidance recommends. Consistency in how things are named across the documentation reduces the reader’s effort, since the same concept under two names reads as two concepts.
The hardest discipline is keeping documentation current. Documentation that contradicts the code is worse than none, because it actively misleads. The sustainable practice is to treat documentation as part of a change rather than a later chore: when behavior changes, the documentation describing it changes in the same commit, and reviewers check for it. In my experience, documentation maintained alongside the code remains trustworthy, while documentation deferred to a future cleanup decays into a liability.
Conclusion
A codebase remains maintainable when its documentation serves each reader at the altitude they need: a system overview for the newcomer, module documentation for the focused contributor, and inline rationale for the maintainer of difficult code. A tested, orienting README provides the entry point; comments that explain why rather than what provide the irreplaceable context; and the practice of updating documentation in the same change that alters behavior keeps the whole trustworthy. Documentation written this way is not overhead but the means by which work survives its author.