  We unify two widely different approaches to understanding the infrared
behaviour of quantum chromodynamics (QCD), one essentially phenomenological,
based on data, and the other computational, realised via quantum field
equations in the continuum theory. Using the latter, we explain and calculate a
process-independent running-coupling for QCD, a new type of effective charge
that is an analogue of the Gell-Mann--Low effective coupling in quantum
electrodynamics. The result is almost identical to the process-dependent
effective charge defined via the Bjorken sum rule, which provides one of the
most basic constraints on our knowledge of nucleon spin structure. This reveals
the Bjorken sum to be a near direct means by which to gain empirical insight
into QCD's Gell-Mann--Low effective charge.
