Schematic illustration of the internal structure of the M 87 jet adopted in the analysis presented in this paper. The superluminal kinematic feature in the jet is associated with the sheath and the subluminal feature is related to a slower, outer layer of the flow. The flow is also expected to feature a fast inner spine, which however cannot be detected in the present data either because of its lower emissivity compared to the emissivity of the sheath layer or because its speed is too high to be detected with the present data.