The Weeknd, a formerly shadowy figure whose songs are dark and sometimes disturbing, has reached the top of the pop charts. He’s part of a new crop of artists who are set to redefine a genre whose limits can be hard to define in the first place. What does their music share with chart-topping R. B. predecessors who range from Aretha Franklin to R. Kelly? And what are they doing that is truly new? Carrie Battan and Kelefa Sanneh join Amelia Lester and David Haglund to discuss what R. B. has been and what it might become.