# Descriptive Geometry/Rotating a line

Why use rotation with lines? In some cases, rotation is a quicker and simpler method than finding an auxiliary view. With rotation, you need only to rotate in one view and bring the information into the other provided view.

Finding AB in True Length

1. Use a compass to measure the length of line AB (in top view). Construct a circle of radius AB, and bring the line parallel to the folding line. (AB1, in T view).
2. Transfer the point B, from T view to F view. Do this by drawing construction lines perpendicular from B1 in T view down to F view.
3. Draw a horizontal line through the original point B in F view until it intersects the perpendicular line you make in step 2. The intersection is B1 in F view.
• Example Problem

Rotating AB about an Axis (while preserving the length)

1. Use a compass to measure the length from A to the Axis (in R view). Construct a circle of radius (A-Axis), and move point A along that circle. The new point is A1 in R view.
2. Transfer the point A1 from R view to F view. Do this by drawing construction lines perpendicular from B1 in R view down to F view.
3. Draw a vertical line through the original point A in F view until it intersects the perpendicular line you made in step 2. The intersection is A1 in F view
4. Now rotate point B while at the same time preserving its length. Do this by using a compass to measure the length from B to the Axis (in R view). Construct a circle of radius (B-Axis).
5. Measure the length AB (in R view) and make a circle of radius AB with the center at the Axis. the point where it intersects the circle with radius (B-Axis) is B1 in R view.
6. Transfer the point B1 from R view to F view. Do this by drawing construction lines perpendicular from B1 in T view across to F view.
7. Draw a vertical line through the original point B in F view until it intersects the perpendicular line you made in step 5. The intersection is B1 in F view.