A study of the nature and function of British Sign Language (BSL) as used in the British deaf community is described. The study examined two hypotheses: (1) that the notion of diglossia applies to the British deaf signing community, and (2) that the low variety of BSL will exploit the visual medium in its grammar to a greater extent than the high variety. Data were collected on the structural features of the language relating to diglossia according to Charles Ferguson's work, and on those areas of grammar sharing special potential for exploitation of the visual medium: spatial modification as a marker of case relations, negatives, and interrogatives. The research population was the entire British signing community of about 40,000. Results show the community has a clear perception of two varieties of BSL, each used under different conditions comparable to high and low forms in diglossia, and that the low form does exploit the visual medium in its grammar to a greater extent. The findings have both theoretical and practical implications. Further support is found for the notion of diglossia and for the relationship of structure and function. In addition, it is proposed that the controversy over the use of signing in schools can now be better informed, the methodology of teaching sign language can be improved and made more explicit, and the expansion of sign language can proceed with more information. Symbols for writing BSL, and a two-page reference list are appended. (MSE)