BOTH allows you to create two lines; one that is parallel 
to the construction-view x axis, and one that is
perpendicular to the construction-view x axis.

Indicate a position for the new line(s), using a Position
Menu option.

In 2-D construction mode, the reference point is projected
to the current construction plane, and the line is created
in the current construction plane.

In 3-D construction mode, the line is created in the plane
that is parallel to the current construction plane and 
that passes through the indicated reference point.
