A common task in geometry is to construct a line through a point that is parallel to a given line. Before starting this tutorial, make sure you have Shows Intersections turned on the Canvas section of Settings.

1. We are given a line (AB), and a point C.

2. Draw a line from C that passes through line AB. This is called a transversal.

3. Tap the Arc tool. The compass should appear in the center of the screen.

4. Drag the center handle of the compass to move it to the intersection of the two lines. Drag the compass arm to rotate it to the position shown.

5. Draw an arc across line AB, then move the compass handle to point C.

6. Draw a similar arc that would intersect the transversal. The arcs should now look like this with a different tool selected to hide the compass:

7. Now we need the intersection of the transversal and this last arc. Tap and hold on the transversal to select it, then tap it to open the menu if it isn’t already. Choose Info, then turn on Extends.

8. Move the compass to the intersection of the transversal and the first arc. Drag the outer handle to resize it. The outer handle should be at the intersection of line AB and the arc.

9. Move the compass to the intersection of the transversal and the second arc. Now draw a *third* arc across the second arc. This marks the second point of the parallel line.

10. Draw a line with the Line tool between point C and this latest point. If you follow the steps above to extend the line, the diagram will finally look like this:

Please leave a comment if you have any questions!