Gravitational waves (GWs) emitted by binary black hole coalescences provide an excellent opportunity to study General Relativity (GR) in its strong field regime. This GW emission consists on a superposition of different GW harmonics. When the black holes are far apart, the so called quadrupolar harmonic dominates the rest, known as higher harmonics (HMs),which only  get triggered during the final merger and ringdown stages of the binary.  In this talk, I will show how higher harmonics can be exploited to decode the properties of the binary and the final merged black hole: from its parameters to the properties of its event horizon.