Brooks's law states that adding people to a late software project makes it later. New team members are not immediately productive: they need time to understand the codebase, domain, and conventions, consuming the time of experienced members who train them.

Communication overhead scales quadratically. A team of n people has n(n-1)/2 potential communication channels. Doubling the team more than doubles coordination cost. Late projects are often blocked by tightly interdependent tasks that cannot be cleanly partitioned, so adding people does not help.

The law is a heuristic, not absolute. Well-partitioned projects with independent modules can benefit from adding people, but only after ramp-up costs are paid. The practical lesson: treat late projects by reducing scope, not adding people.