The numerical notation associated with texts and other representational media used in ancient societies is an important means by which past cognitive processes may be reconstructed. No satisfactory typology exists, however, to help understand the relationship between numerical symbols and cognitive processes. As a result, theories concerning the development of numeration remain mired in a unilinear and ethnocentric framework in which our own (Hindu-Arabic or Western) numerals are seen as the ultimate stage of evolution. It is suggested herein that there are two separate dimensions that need to be considered when classifying and evaluating numerical notation systems, and that these dimensions are structured in highly constrained ways. A new typology is presented in which systems are classified into five major types on the basis of these dimensions. Using this typology, a multilinear model is presented for the patterned diachronic change in numerical notation systems, which refutes both unilinear evolutionary theories and radically relativistic propositions regarding how individuals in pre-modern societies represented numbers.