Mass plane diagram for Cyg X-1, showing component masses and their uncertainties. BH masses are estimated for i = 27.5^°(blue) and i = 33.7^° (orange).The solid black line represents the mass relation for the adopted inclination of i = 27.5^° derived from optical observations (Orosz et al. 2011; Miller-Jones et al. 2021). The dotted line corresponds to i = 33.7^° derived assuming synchronous rotation. The dashed lines illustrate the variation in the mass relation for alternative inclination angles.