Abstract
Polycentrism is seen as a desirable model of metropolitan structure. The starting point in its study rests upon the conceptualisation and identification of sub-centres. The hegemonic family of sub-centre identification is based on the analysis of density. Huge efforts have been devoted to achieving statistically robust models, but little attention has been paid to the conceptualisation of density itself. This paper presents a different approach to calculating density through the introduction of basic elements of mobility and placing priority on sub-centres as those municipalities that are closer to the urban paradigm of central-cities in southern-Europe -they are attractive both for living and working. The analysis in Barcelona and Madrid suggests that compound-density allows for selecting what local experts would define as a sub-centre, discarding most dormitory, adjacent-to-CBDs and small municipalities, found by classical-density. Furthermore compound density sub-centres are denser, they contain more central-activities and are strongly linked to other municipalities.