  Classical phase-space variables are normally chosen to promote to quantum
operators in order to quantize a given classical system. While classical
variables can exploit coordinate transformations to address the same problem,
only one set of quantum operators to address the same problem can give the
correct analysis. Such a choice leads to the need to find the favored classical
variables in order to achieve a valid quantization. This article addresses the
task of how such favored variables are found that can be used to properly solve
a given quantum system. Examples, such as non-renormalizable scalar fields and
gravity, have profited by initially changing which classical variables to
promote to quantum operators.
