  The inner parts of black-hole accretion discs shine in X-rays which can be
monitored and the observed spectra can be used to trace strong gravitational
fields in the place of emission and along paths of light rays. This paper
summarizes several aspects of how the spectral features are influenced by
relativistic effects. We focus our attention onto variable and broad emission
lines, origin of which can be attributed to the presence of orbiting patterns
-- spots and spiral waves in the disc. We point out that the observed spectrum
can determine parameters of the central black hole provided the intrinsic local
emissivity is constrained by theoretical models.
