GW190521 is a significant discovery due to the masses of the resulting large black hole, and one or both of the smaller constituent black holes. Only indirect evidence for intermediate mass black holes, those in the range 102-105 solar masses, had been observed earlier, and it was unclear how they had formed.[14] Researchers hypothesize "a hierarchical merger, in which the two progenitor black holes themselves may have formed from the merging of two smaller black holes, before migrating together and eventually merging."[2]