  Recent cognitive experiments have shown that the negative impact of an edge
crossing on the human understanding of a graph drawing, tends to be eliminated
in the case where the crossing angles are greater than 70 degrees. This
motivated the study of RAC drawings, in which every pair of crossing edges
intersects at right angle. In this work, we demonstrate a class of graphs with
unique RAC combinatorial embedding and we employ members of this class in order
to show that it is NP-hard to decide whether a graph admits a straight-line RAC
drawing.
