  I discuss the effect of the formation of a black hole on a (close) binary and
show some of the current constraints that the observed properties of black hole
X-ray binaries put on the formation of black holes. In particular I discuss the
evidence for and against asymmetric kicks imparted on the black hole at
formation and find contradicting answers, as there seems to be evidence for
kick for individual systems and from the Galactic $z$-distribution of black
hole X-ray binaries, but not from their line-of-sight velocities.
