 to the bed, with perfect arms and end effector linkages,
this is not a problem.  However, in practice, there are always
construction imperfections.
These imperfections can cause distortions in the XY plane.
It can sometimes be better to have calibrated parameters which sacrifice
bed flatness for low distortion (higher accuracy) in the XY plane.
The residual bed-flatness error can be removed by an automatic
bed-leveling procedure (a.k.a. bed tram compensation).

<P>See my earlier
<A HREF="Calibration.html">