Schematic plot of a wind-driven bubble model. Region a is the free-wind region filled with hypersonic stellar winds. Region b is the shocked-wind region, which sets the inner boundary for the H II region. Region c is composed of shocked gas. Region d is the H II region. Depending on the ionization rate, region d can be trapped within region c or extend beyond it. Region e is the ambient neutral medium. The relative sizes of the regions are not to scale.