  We present a model independent analysis of custodial corrections to the $S$
parameter. The negative contributions coming from direct corrections, i.e. the
corrections associated to effects of new physics in the fermionic sector, can
be used to eliminate unwanted positive oblique contributions to $S$. By means
of such an ideal cancellation among oblique and direct corrections the
electroweak physics can be made insensitive to custodial new physics. The
Lagrangian analysis of the ideal cancellation reveals a possible sizable
redefinition of the effective electroweak energy scale with respect to models
with only oblique corrections. We then infer from general principles the full
expression for the effective custodial operator responsible in the gauge sector
for the contribution to $S$. We show that the ideal cancellation exactly
eliminates all the custodial corrections, including those in the non-abelian
and longitudinal terms. Indeed, provided redefinitions of the physical
parameters, the standard model Lagrangian can be equivalently defined modulo
custodial corrections in the gauge and fermionic sectors. Thus the ideal
cancellation can be regarded as an effective symmetry. Finally we investigate
the theoretical origin of this effective symmetry in terms of the effective
gauge structure induced by custodial new physics.
