Halliday & Matthiessen (1999: 613):
The protolanguage is typically associated with the stage of crawling, when children are mobile, but not yet walking and running: typically about 0;8 - 1;4, but with wide variation around these times. The elements of the protolanguage are "signs" (that is, content/ expression pairs); they are thus formally identical with the semiotic resources of higher mammals (primates and cetaceans) — but with one important difference: the signs of other species become codified as the form of communication among adults, whereas those of human children are transitional to a system of a different kind, and hence do not stabilise into a settled pattern but are constantly shifting on both semiotic planes.