Illustration of the binary stellar evolution leading to a BH+BH merger with a high chirp mass. The initial metallicity is Z_⊙/ 50, the masses of the stars in solar masses are indicated with red numbers, and the orbital periods in days are given as black numbers. A phase of contact near the ZAMS causes mass exchange. Acronyms used in the figure: ZAMS: zero-age main sequence; TAMS: termination of hydrogen burning; He-star: helium star; SN: supernova; GRB: gamma-ray burst; BH: black hole.