Musicians playing together can have all the chops, musical ideas, and swagger in the world, but they aren't a band until they have chemistry. It's not the lead singer, guitarist, or any other individual that audiences respond to; it's the way those elements mix, bubble, explode, and so forth. Time and again in rock from the '60s to the '90s, adding a new member to a band completely shifted that chemical reaction, shaking up their sound and forever transforming fortunes and legacy.