Along with density, mix and heterogeneity are elementary components to produce an IDEAN urban fabric that will produce an intense urbanity. Indeed, the question of the right quantity, quality, and scale of mix is one of the core questions of good urban design. Ancient and traditional cities feature a highly mixed and heterogeneous urban fabric. At the same time they were based on a rigorous and impenetrable social order of class, caste, guild, and/or ethnicity. The modern industrial metropolis aims for a segregation of industry, housing, leisure facilities and commerce, as well as large scale land use. IDEAN, then, grounded in the age of information and post-fossil fuels and the principles of participation and democracy, promotes a model of mix and heterogeneity based on the scale of the block. This acknowledges the need for collective organisation and voluntary clustering of minority populations, while at the same time avoiding socio-spatial ruptures at larger dimensions. Mix and heterogeneity are closely related to distinct aspects of urban cultures such as social learning and conviviality.