“Visual meteorological conditions” (v.m.c.) specifies a forward visibility of at least, say, 5 miles, horizontal distance from cloud of at least, say, 2000 ft., and vertical distance from cloud of at least, say, 500 ft. At any instant, therefore, the airspace may comprise (a) v.m.c. areas (or strictly speaking volumes), and (b) non-v.m.c. areas. The dividing lines (or strictly speaking surfaces) between them may be perceptible or their positions predictable by the aircrew or they may not. If they are neither perceptible nor predictable, then the aircrew cannot possibly ensure that their aircraft does not enter any non-v.m.c. area and it cannot possibly ensure compliance with instructions to ‘maintain v.m.c.’. Whether the aircraft is in a v.m.c. area or in a non-v.m.c. area may be perceptible by the aircrew or it may not. If it is perceptible and if the aircraft immediately reverses direction in a small space, then the aircrew will almost but not quite be able to ‘maintain v.m.c.’. In some cases very small infractions of non-v.m.c. areas may be permissible; in others, not.