One of Illustrator’s many potent tools, and something of an unsung hero, is the Width Tool. Nested in with some of the other exotic tools, like Warp and Bloat, the Width Tool allows us to vary the thickness of a stroke at any given point along a path.
Simply select the Width Tool then click-drag at the desired point on a path (active or inactive) and you’ll see control handles appear and the stroke width will increase as you drag outwards.
For even more expressive strokes hold down the Alt/Opt key to alter the thickness on one side of the stroke at a time or add additional control points to vary the thickness at different intervals. Once the control points are in place you can select and drag to reposition them anywhere along the path. To create sharp transitions in thickness the control points can be dragged to butt-up against each other.
The Width Tool really doesn’t take long to master but enables you to quickly turn static artwork into more interesting and expressive graphics with just a few clicks.