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The tree structure is the hierarchical structure that organises the contents and features of a digital product. It defines which sections exist, how they are grouped and how they nest within one another. It is the foundation of navigation: if the tree structure is inconsistent, no interface can make up for it. It is one of the first decisions to make in information architecture.

You start with a card sorting session with representative users to understand how they naturally categorise contents. You build a candidate tree structure from this data, then validate it with a tree test. You iterate until users find contents without hesitation. The final tree structure must reflect users' mental models, not the internal organisation of the company.

There is no universal rule, but beyond 3 levels of depth, users begin to struggle to keep track of their position in the hierarchy. The classic tension is between breadth (many categories at the first level) and depth (few categories, more levels). Studies show that broad (but shallow) tree structures are generally easier to navigate than deep tree structures.

The tree structure is the underlying structure; navigation is the way it is made visible and accessible in the interface. The same tree structure can be presented through a hamburger menu, a horizontal navigation bar, a mega-menu or a side panel. The choice of navigation pattern depends on the volume of contents, the uses (mobile vs. desktop) and the conventions of the target platform. But it is always the tree structure that determines what navigation exposes.

By integrating new contents into the existing structure rather than adding ad hoc sections. A tree structure that grows without logic quickly becomes a maze. You need to audit rarely visited sections regularly (via analytics), record internal searches that indicate users are not finding what they are looking for, and question the structure if usage patterns drift away from the logic of the original tree structure.