1. When we rotate the camera, we observe different combinations of stresses in the pipe. The stresses that we originally measured in the x and y directions (i.e., the calibration stresses) did not change. The only thing that changed was our viewing orientation.

2. We've discovered that there are compression stresses in some directions even though there were no compression stresses in our original x and y calibration stresses.

3. Suppose our pipe is made from of a material that ruptures at a stress of 80? If we had considered only the x and y direction stresses, we would have thought that the pipe was safe. However, the pipe would have ruptured perpendicular to the direction of the largest blue arrow. As designers, we cannot be content knowing the state of stress in only the x and y directions...we must know the state of stress in all directions.