Hate giving cred to Chicago...but I bow to Chi Lites
The Chi Lites were a R&B band out of Chicago in the early 1970's that were credited with... of all things, augmenting the "Philly" sound (whatever that is, I think of Sly Stone, of "Sly and the Family..."), but categorizing is done at one's own peril.
I just know that the vocals and harmonizing are s-o-o outstanding that it merits big-time recognition.