Supermassive black holes are classically defined as black holes with a mass above 0.1 million to 1 million M☉.[9] Some astronomers have begun labeling black holes of at least 10 billion M☉ as ultramassive black holes.[10][11] Most of these (such as TON 618) are associated with exceptionally energetic quasars. Even larger ones have been dubbed stupendously large black holes (SLAB) with masses greater than 100 billion M☉.[12] Although they noted there is currently no evidence that stupendously large black holes are real, they noted that supermassive black holes almost that size do exist.[13] Some studies have suggested that the maximum mass that a black hole can reach, while being luminous accretors, is of the order of ~50 billion M☉.[14][15]