  In this manuscript, we show that three fundamental building blocks are
supporting the Cosmological Principle. The first of them states that there is a
special frame in the universe where the spatial geometry is intrinsically
homogeneous and isotropic. The second demands the existence of a fiducial
observer to whom the Hubble parameter is isotropic. The last piece states that
matter and radiation behave as a perfect fluid. We show that these three
hypotheses give us the Friedmann-Lema\^itre-Robertson-Walker (FLRW) spacetimes,
the central pillar of the standard model of Cosmology. We keep with the first
of them and start to investigate the so-called intrinsically homogeneous and
isotropic spacetimes. They emerge after the decoupling of the CMB with the
geometric frame of reference. Furthermore, a ``$\Lambda$CDM-like" effective
theory arises naturally in those backgrounds, together with some new density
parameters relating to the local inhomogeneities, the internal energy density,
and the local and global magnitudes of the Hubble anisotropy. All those
properties make this class of inhomogeneous models, which roughly speaking,
keeps "1/3" of the Cosmological Principle, worth investigating in applications
to Cosmology, for it can accommodate the same ingredients of the standard
model, as a geometric frame and a free-falling isotropic cosmic background
radiation, and reduce to the latter when some observable parameters vanish.
