Massive black holes are located in the centre of all massive galaxies. During the encounter of galaxies those black holes pair and, after some hardening period, become sources of gravitational waves (GWs).  Binaries at the early stage of inspiral emit almost monochromatic GWs and the population of massive black hole binaries (MBHBs) is the main source for the Pulsar Timing Array (PTA).  In PTA we use radio emission from the millisecond pulsars to detect GWs in the nano-Hz frequency band.