Schematic illustration of two possible positions of the masing gas relative to the H ii region with radius R. For case A the centre of the masing region, with linear dimension of 1.5R, is at a distance 2R from the centre of the H ii region which corresponds to |$W_{\rm H\, {{\small II}}}$|  = 0.07. At the right-hand end of the masing volume |$W_{\rm H\, {{\small II}}}$|  = 0.06. In case B, the masing region is projected against the H ii region such that the left-hand end is at distance 2R from the centre of the H ii region. The right-hand end is at 3.5R from the centre of the H ii region where |$W_{\rm H\, {{\small II}}}$|  = 0.02.