‘Leave no one behind’ has become an important principle of the entire 2030 Sustainable Development Agenda and the Sustainable Development Goals that brings inequality to the centre stage. Although a precise definition of what it is to lag behind is essential to assess sustainable economic growth and social progress a discussion on the desirable properties of such a measure is absent in the literature. This paper fills that gap by proposing and discussing a number of normative and analytical properties a measure of the ‘Leave no one behind’ principle should satisfy. The axioms proposed are necessary and sufficient to characterize the fuzzy measure by García-Pardo et al. [8] and together with other additional axioms make the structure of this measure more transparent.