How did Chicago drill music develop from trap music?

Chicago drill music emerged as a distinct style that grew directly from trap music, an earlier subgenre of gangsta rap that originated in Atlanta during the 1990s.1

The term drill was coined by the Chicago underground rapper Pac Man to describe this evolving sound. While drill maintains the foundational influence of trap, it is characterized by its own specific stylistic markers, most notably a monotone vocal delivery and lyrics that are explicitly aggressive in nature.1

Following its development in Chicago, the genre gained wider recognition through the work of local artist Chief Keef and was subsequently adopted by mainstream musicians such as Kanye West and Drake.1