The first black-hole merger detected through gravitational waves was GW150914.
LIGO observed GW150914 on 14 September 2015, and the collaboration announced the discovery on 11 February 2016. The signal came from two stellar-mass black holes spiraling together and forming a single larger black hole. It was the first direct detection of gravitational waves and the first observed merger of two black holes.
The wave pattern began at low frequency as the objects orbited, rose in pitch during the final inspiral, peaked at merger, and then faded during ringdown. This characteristic shape matched general-relativistic predictions.
The two original black holes had estimated masses of about 36 and 29 Suns. The remnant was about 62 solar masses, with roughly three solar masses converted into gravitational-wave energy. GW150914 is often confused with GW170817, which was the first gravitational-wave event observed from merging neutron stars.