  We present the final combination of CDF and D0 measurements of cross sections
for single-top-quark production in proton-antiproton collisions at a
center-of-mass energy of 1.96 TeV. The data correspond to total integrated
luminosities of up to 9.7 fb$^{-1}$ per experiment. The $t$-channel cross
section is measured to be $\sigma_t = 2.25^{+0.29}_{-0.31}$ pb. We also present
the combinations of the two-dimensional measurements of the $s$- vs.
$t$-channel cross sections and of the $s+t$ channel cross section measurement
resulting in $\sigma_{s+t} = 3.30^{+0.52}_{-0.40}$ pb, without assuming the
standard-model value for the ratio $\sigma_s/\sigma_t$. The resulting value of
the magnitude of the top-to-bottom quark coupling is $|V_{tb}|$ =
$1.02^{+0.06}_{-0.05}$, corresponding to $|V_{tb}| > 0.92$ at the 95% C.L.
