The symmetry line for a mirror constraint may be any linear element.

Complete only extracts mirrored pairs from areas that are orthogonal to defined symmetry lines. If your symmetry lines have infinite length (e.g., construction lines) this is no limitation, but if you choose a finite-length element as a symmetry line, items must lie completely within the orthogonal boundary in order to be selected for mirroring. The two line segments on the right as shown in the below figure, for example, would not be extracted because neither falls inside the boundary. If you are assigning mirrored pairs manually, this limitation does not apply.