  The discrepancy between the value of the Hubble constant $H_0$ in the late,
local universe and the one obtained from the Planck collaboration representing
an all-sky value for the early universe reached the 5-$\sigma$ level.
Approaches to alleviate the tension contain a wide range of ansatzes:
increasing uncertainties in data acquisition, reducing biases in the
astrophysical models that underly the probes, or taking into account
observer-dependent variances in the parameters of the cosmological background
model. Yet, early and late universe probes are often treated as independent,
they live on different length scales, and require different perturbations to be
subtracted. Hence, fitting a flat Friedmann-Lema\^itre-Robertson-Walker
cosmology to different probes at different cosmic epochs can yield different
sets of cosmological parameter values. Tensions arise if these background fits
and perturbing biases are not consistently calibrated or synchronised with
respect to each other. This consistent model-fitting calibration is lacking
between the two $H_0$ values mentioned above, thus causing a tension. As shown
here, this interpretation resolves the $H_0$tension, if 15% of the
matter-density parameter obtained from the fit to the cosmic microwave
background, $\Omega_m = 0.315$, are assigned to decoupled perturbations
yielding $\Omega_m = 0.267$ for the fit at redshifts of the supernova
observations. Existing theoretical analyses and data evaluations which support
this solution are given.
