  This work is a continuation of two previous papers of a series, in which we
examined the pulse-width statistics of normal radio pulsars. In the first paper
we compiled the largest ever database of pulsars with interpulses in their mean
profiles. In the second one we confirmed the existence of the lower boundary in
the scatter plot of core component pulse-widths versus pulsar period W50 sim
2.5 P^{-0.5}[deg], first discovered by Rankin using much smaller number of
interpulse cases. In this paper we show that the same lower boundary also
exists for conal profile components. Rankin proposed a very simple method of
estimation of pulsar inclination angle based on comparing the width W50 of its
core component with the period dependent value of the lower boundary. We claim
that this method can be extended to conal components as well. To explain an
existence of the lower boundary Rankin proposed that the core emission
originates at or near the polar cap surface. We demonstrated clearly that no
coherent pulsar radio emission can originate at altitudes lower than 10 stellar
radii, irrespective of the actual mechanism of coherence. We argue that the
lower boundary reflects the narrowest angular structures that can be
distinguished in the average pulsar beam. These structures represent the core
and the conal components in mean pulsar profiles. The P^{-0.5} dependence
follows from the dipolar nature of magnetic field lines in the radio emission
region, while the numerical factor of about 2.5 deg reflects the curvature
radius of a non-dipolar surface magnetic field in the partially screened gap
above the polar cap, where dense electron-positron plasma is created. Both core
and conal emission should originate at altitudes of about 50 stellar radii in a
typical pulsar, with a possibility that the core beam is emitted at a slightly
lower heights than the conal ones.
