Halliday & Matthiessen (1999: 533):
From one point of view, as we introduced it in the earlier discussion, Sign will appear as a realisational variant: that is to say, the meanings of language — its semantic system — may be construed either in sound or in gesture. There are, in fact, sign systems that are constructed along these lines: in principle this is what is meant by "signed English". But if we consider the nature of the two forms of expression, vocal on the one hand and gestural on the other, it is clear that they have very different properties. Gesture operates in a 3-dimensional "signing space" defined by reference to the signer's body and its parts, and movement within that space is entirely accessible to the receiver, thus in addition to succession in time (which is common to both), the gestural medium can exploit a number of parameters of spatial variation: the "articulatory organs" (fingers, hands, arms, other body parts), their location, orientation, thrust (direction and speed of movement) and so on.