The rebus principle refers to the process whereby a logograph normally associated with a particular word is applied to a lexically unrelated but homophonous word, for example, in English, a picture of an eye representing the word "I". This strengthens the phonetic aspect of a writing system by exploiting phonetic similarities. Potentially, this can be extended so that a picture of an eye comes to represent the sound [ai]. The Bamum script employed this process in its initial developments. A monosyllabic language such as Bamum lends itself to a syllabic script, and in the script's final developments the rebus principle was transferred to individual syllables, so that logographs become associated with an abstract but stable sound value rather than with a particular word. About 40 years after first being devised, the Bamum script had transformed from a logography with an inventory of over 500 symbols to a syllabary comprising just 80 basic symbols.
Coulmas, Florian. 1999. The Blackwell Encyclopedia of Writing Systems. Blackwell Publishing. Oxford.