To open the Taper tool, double-click on "Taper" in the Toolbox.
Enter percentage values in the Taper Factor control to taper any selected points along the current Active Axis. Consider the following object:
Here is the object after setting a Taper Factor of 0.33, with Displace Y enabled and the X Axis Active:
Note that the points in the object have been progressively scaled along the X Axis, and that only their Y coordinates have been scaled. If you enable Displace Z, youll see the following:
Now, both Z and Y coordinates have been scaled up to 33 percent along the X axis. Note that the scaling increases based on each points distance from the object Origin. If you set the Origin to "X Min", youll see the following (now flat-viewed along the Z Axis).
To the right of Taper Factor is a ControlChannel, allowing you to alter how the points are scaled, based on their distance from the Taper Origin. Note the following change to the ControlChannel, and its effects on the resulting tapering:
To interactively taper selected points, LMB Drag in any Camera View while Taper is the Current Tool. As you drag the mouse, any selected points will be tapered along the Active Axis.