The default right mouse button option, Select pairs, allows you to select a pair of 2D straight edges, such as lines. The Parallel constraint forces the selected lines or major axes for ellipses and elliptical arcs to be parallel. The right mouse button option, Select multiple, allows you to select a continuing series of lines or ellipses. In the series, after you have selected two edges, a constraint is created and then the second edge you selected is used as the first edge for the next pair. This continues until you either use the right mouse button to start a new series or return to standard pairs selection. A series of these may be preselected before selecting the function and they are treated like a series selected in 'Select multiple' mode.