G-CO.12 Construction #5 Constructing Perpendicular Lines
Geometric Constructions using a compass and a straightedge.
1.  Draw line AB.
2.  Put the compass point on A so that the compass measurement is over halfway to point B.  Draw a long arc above and below the line.
3.  Using the same compass measurement as step 2, place the compass on B and draw a long arc above and below the line so that it intersects the arc in step 2 at C and D.
4.  Draw line CD.
5.  Line AB line CD.