  Relativistically blurred reflection from the accretion disc provides a
powerful probe of the extreme environments close to supermassive black holes;
the inner regions of the accretion flow and the corona that produces the
intense X-ray continuum. Techniques by which the geometry and extent of the
corona can be measured through the observed X-ray spectrum are reviewed along
with the evolution in the structure of the corona that is seen to accompany
variations in the X-ray luminosity both on long and short timescales. Detailed
analyses of the narrow line Seyfert 1 galaxies Markarian 335 and 1H0707-495,
over observations with XMM-Newton as well as Suzaku and NuSTAR spanning nearly
a decade reveal that increases in the X-ray luminosity coincide with an
expansion of the corona to cover a larger area of the inner accretion disc.
Underlying this long timescale variability lie more complex patterns of
behaviour on short timescales. Flares in the X-ray emission during a low flux
state of Mrk 335 observed in 2013 and 2014 are found to mark a reconfiguration
of the corona while there is evidence that the flares were caused by a vertical
collimation and ejection of coronal material, reminiscent of an aborted
jet-launching event. Measurements of the corona and reflecting accretion disc
are combined to infer the conditions on the inner disc that lead to the flaring
event.
