Halliday & Matthiessen (1999: 533):
In the most immediate sense, as regards their potential for construing signifiers — elements of wording and their arrangement in combination — both these forms of expression, vocal and gestural, are open-ended. Neither of them imposes a limit on the inventory of morphemes or their configuration in grammatical structures. Nevertheless they are significantly different in the kinds of resource they offer for making meaning. Perhaps the major difference between the two is their potential for iconicity.