Halliday & Matthiessen (1999: 534-5):
Signers are also members of another language community, that of the (predominantly hearing) speakers of English, or whatever language is spoken around them; the two groups interact, and there is obviously no insulation between the two language systems. This gives rise to contact phenomena of two kinds: on the one hand, intermediate forms whereby English is realised in sign expressions (signed English, and finger-spelling), including a large number of new, "contrived" signs; and on the other hand, constant intrusion of English forms of expression, and therefore of English modes of meaning, into the sign language itself.