In approximately 10% of active galactic nuclei (AGN) the system of a black hole and an accretion disk is capable of launching powerful relativistic plasma jets, which in some cases have enough energy to propagate outside the host galaxy. Traditionally, the signature of relativistic jets is strong radio emission that is due to synchrotron radiation. Conversely, the relativistic jet is typically less dominant in the optical band. For these reasons, the radio-loudness criterion introduced by Kellermann et al. (1989), defined as the ratio between the flux density at 5 GHz and the optical B-band flux density, is usually good at separating sources with and without relativistic jets. When the ratio is higher than 10, the source is classified as radio-loud. If the ratio is lower than 10, the source is radio-quiet. The power and luminosity of relativistic jets depend on the black hole mass and follow a nonlinear scaling relation. This means that low-mass AGN inevitably harbor less powerful jets (Heinz and Sunyaev 2003; Foschini 2014). Because the scaling weighs the radio luminosity higher than in the optical, the radio-loudness criterion can become inadequate in low-mass sources.