Julia Barham's framework for developing well-rounded product leaders: rather than specializing narrowly in one phase or one kind of product, a PM should deliberately collect 'tours of duty' across both discovery and delivery work, and across different points in the Product Maturity Lifecycle (0→1, 1→n, At Scale) — zero-to-one, scaling, and at-scale/optimization work. The toolkit accumulated this way becomes essential once a PM rises into leadership, where they're no longer running a single backlog but managing a varied portfolio of products at different maturity stages (see Four Slices of the Pie (Product Portfolio Balance)) — narrow specialization leaves gaps exactly where that portfolio view is needed most.