Horizontal codebase structures that group code by type (components, hooks, utils, types) create poor cohesion and make large codebases hard to navigate. The alternative is a vertical structure that groups code by domain or feature — everything related to 'widgets' lives in src/widgets/, regardless of whether it's a component, hook, or utility. This mirrors how product teams are organized and reduces cognitive load. Shared code that spans multiple features becomes its own vertical. To enforce boundaries between verticals, tools like pnpm workspaces, Nx dependency rules, or eslint-plugin-boundaries can define public interfaces and prevent unintended coupling. The tradeoffs include difficulty choosing the right vertical and risk of duplicated implementations across teams.