Published online by Cambridge University Press: 24 October 2008
1. Introduction. Abstract length functions were first considered axiomatically in Lyndon (4). Length functions satisfying a less restrictive set of axioms were studied by Chiswell(1). Groups with a normal form structure (NFS-groups) were studied in Hurley (3) using a slightly different axiomatization to that used in this paper. Length functions naturally arise from normal form structures. In this paper we show that the length functions arising from normal form structures can be characterized axiomatic-ally by a set of axioms that consists of those of (1) together with one further axiom.