  We propose a definition for background (in)/dependence in dynamical theories
of the evolution of configurations that have a continuous symmetry and test
this definition on particle models and on gravity. Our definition draws from
Barbour's best-matching framework developed for the purpose of implementing
spatial and temporal relationalism. Among other interesting theories, general
relativity can be derived within this framework in novel ways. We study the
detailed canonical structure of a wide range of best matching theories and show
that their actions must have a local gauge symmetry. When gauge theory is
derived in this way, we obtain at the same time a conceptual framework for
distinguishing between background dependent and independent theories. Gauge
invariant observables satisfying Kuchar's criterion are identified and, in
simple cases, explicitly computed. We propose a procedure for inserting a
global background time into temporally relational theories. Interestingly,
using this procedure in general relativity leads to unimodular gravity.
