Halliday (2008: 140-1):
When we look at it “from below”, in terms of its mode of expression, the complementarity is very obvious: speech happens, as ongoing transitory disturbances in the air, that we recognise as sound waves; writing exists, as simultaneous and relatively permanent visible marks, on stone or metal or vegetable matter processed into paper. It is only in the last few decades that these properties have come to be mixed together, so that we can now “capture” spoken language on tape, and “scroll” written language up (but also down) the computer screen. We don’t yet know what are going to be the longer-term consequences of this interpenetration of these two modes of expression, which were previously much more distinct. But changing pressures from below — and also those from above — are bound to have some significant effects.