The Curve ruler's name is a little bit misleading because it can be used to make polyline straight-line rulers, as well as rulers with smooth curves, as shown in the following screenshot:
To use the Curve Ruler, select it from the Ruler sub tool category. In the Tool Property palette, you will see four options under the Curve setting. The very first icon will produce straight lines between the points of your ruler, and the second option is the Spline setting, which will create curves between each point of the ruler. To use either of these options, simply click on the point on the canvas where you want the ruler to start. Then, ...
