  We discuss a lattice QCD computation of the $B$-meson decay constants by the
ETM collaboration where suitable ratios allow to reach the bottom quark sector
by combining simulations around the charm-quark mass with an exactly known
static limit. The different steps involved in this ratio method are discussed
together with an account of the assessment of various systematic effects. A
comparison of results from simulations with two and four flavour dynamical
quarks is presented.
