  We present a critical analysis of the usual interpretation of the multicolour
disc model parameters for black hole candidates in terms of the inner radius
and temperature of the accretion disc. Using a self-consistent model for the
radiative transfer and the vertical temperature structure in a Shakura-Sunyaev
disc, we simulate the observed disc spectra, taking into account doppler
blurring and gravitational redshift, and fit them with multicolour models. We
show not only that such a model systematically underestimates the value of the
inner disc radius, but that when the accretion rate and/or the energy
dissipated in the corona are allowed to change the inner edge of the disc, as
inferred from the multicolour model, appears to move even when it is in fact
fixed at the innermost stable orbit.
