The part was created in the assembly, and uses the assembly for its references. In other words, when making the first sketch for the part (Sketch1), it was done within the assembly. So the references for this sketch were datum planes and surfaces from the assembly. When I opn this created part and try to redefine the references in Sketch1, there is nothing else available to select as references. I can create part datum planes, but they have to be inserted after the first sketch, so they do no good. I cannot create datum planes before the fisrt sketch in the model tree. I would like to create a new, independent part definition.