GW 190412 was a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 12 April 2019.
[1][2] In April 2020, it was announced as the first time a collision of a pair of very differently sized black holes has been detected.
[3] As a result of this asymmetry, the signal included two measurable harmonics with frequencies approximately a factor 1.5 (a perfect fifth) apart.
[2][4] The collision took place 2.4 billion light-years away.
[3] The difference in mass meant that the secondary harmonic in the signal was strong enough to be detected, allowing researchers to perform a test of general relativity and determine that the larger black hole was spinning.