A pair. A duo. A twosome. That’s how everybody thinks about Emily Saliers and Amy Ray: They just naturally go together.

Indeed, it points out the obvious to note they’ve notched nearly four decades of success as singer-songwriters, thanks to their incredible collaborative chemistry. Not for nothing, one of their most memorable songs is “Power of Two” (even if those lyrics are about a romantic rather than an artistic union).

But here’s the thing about the Indigo Girls—their ineffable alchemy cannot be easily explained. As two individuals who craft magic together, they exemplify the arithmetic-defying adage: Their whole is greater than the sum of their parts.