  We discuss the question of gauge choice when analysing relativistic density
perturbations at second order. We compare Newtonian and General Relativistic
approaches. Some misconceptions in the recent literature are addressed. We show
that the comoving-synchronous gauge is the unique gauge in General Relativity
that corresponds to the Lagrangian frame and is entirely appropriate to
describe the matter overdensity at second order. The comoving-synchronous gauge
is the simplest gauge in which to describe Lagrangian bias at second order.
