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Secure measurements of the mass of the central supermassive black hole, MBH, in external galaxies are traditionally obtained through the modeling of the stellar and/or gaseous kinematics, most often derived using Hubble Space Telescope (HST) observations in the optical domain. The modeling of the nuclear ionized-gas kinematics has led to accurate MBH measurements at a relatively cheap cost in terms of observation time compared to stellar-dynamical MBH determinations. But only a handful of the objects have turned out to have sufficiently regular gas velocity fields for the purpose of modeling. Nevertheless, the HST archive contains a yet untapped resource that can be used to better constrain the MBH budget across the different morphological types of galaxies, which consists of the vast number of the Space Telescope Imaging Spectrograph (STIS) spectra from which a central emission-line width can be measured. These data allow to put an upper limit on MBH for a large number of galaxies and promise to compensate for the lack of exact measurements when studying the MBH–host galaxy relationships.