The design of a natural ventilation strategy requires the establishment of the location and size of a series of purpose provided ventilation openings (PPOs). The success of the design is dependent on knowledge of the aerodynamic performance of the PPOs often described by their geometry (normally an area) and resistance to airflow. The incorrect interpretation of this information can lead inappropriate ventilation strategies and buildings that overheat and have an excessive energy demand. Many definitions of PPO area are used by standards, guidelines, text books, and software tools. Each can be assigned multiple terms and a single term can be assigned to different definitions. There is evidence that this leads to errors in practice. An effective area of a PPO, defined as the product of its discharge coefficient and its free area, is proposed as a standard description because it is unambiguous and its measurement is governed by recognised standards. It is hoped that PPO manufacturers will provide an effective area as standard and that its use will be recognised as best practice. It is intended that these steps will reduce design errors and lead to successful natural ventilation strategies and better buildings