When selecting areas for a goal of representing all British breeding birds, the Welsh grid cell SH60 (with the black spot) was chosen because it contributes species including the Chough (listed in the window on the right). For each selected area, it is then possible to plot maps of species richness in just these goal-essential species. In effect, these are maps of flexibility for the selected areas within a particular set (ref 13). Any one of the other areas on these maps that has all of the same goal-essential species as the selected area under consideration (shown in red in the example below) is therefore a perfectly flexible alternative. Thus flexibility can be exploited to find a modified set of areas that is equally efficient and equally easily justified. Areas with fewer of the goal-essential species (shown in dark blue) are only partially flexible, so that choosing them in place of the first area will reduce efficiency (below):