  A realistic study of the physics reach of a t-tbar threshold scan at a future
e+e- linear collider is presented. The results obtained take into account
experimental and, to a large extent, theoretical systematic errors, as well as
beam effects. Because of the large correlations between the physical parameters
that can be extracted from the threshold scan, a multi-parameter fit is seen as
mandatory. It is shown that the top mass, the top width and alpha_s(M_Z) can be
extracted simultaneously with uncertainties around 20 MeV, 30 MeV and 0.0012,
respectively, while the top Yukawa coupling can be measured, with the previous
three parameters, to an uncertainty of about 35%, after assuming an external
prior on alpha_s of +/-0.001.
