# 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**

- 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).
- 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.
- 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)**

- 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.
- 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.
- 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
- 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).
- 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.
- 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.
- 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.