Pen tool
In graphic design software, the pen tool creates vector graphics using mathematical points and curves. Unlike the brush tool, which changes the color value of pixels, the pen tool creates mathematical objects that represent curves and shapes. The style of these objects can be changed before they are rasterized to pixels.
Anchor points and paths
Anchor points are points, connected in a sequence, that determine the vectors of a curve. Usually, the curve passes directly through every anchor point. The curve is called a path.
An anchor point has two handles, which can be adjusted together or independently, to alter the path slope as it passes through the point. If the handles are parallel, the curve passes smoothly through the point (the slope is continuous). If the handles form an angle, the curve is "jagged" at that point (the slope is discontinuous).
Using the pen tool
Pressing P selects the eyedropper tool in Adobe Photoshop.
Click the pen tool on a blank area of a document to create an anchor point. If no other anchor points were selected, the anchor point starts a new path. Click again to create the next point in the path. Press Enter to stop adding points to the path, or click the pen tool on the starting anchor point to close the path. A closed path has an inside and an outside.
If the pen tool is clicked and dragged, it creates a point and also adjust the point's handle, changing the slope of the curve.
If the pen tool is clicked on an existing curve section, a new anchor point is inserted there. If the pen tool is clicked on an existing anchor point, the point is deleted.
Don't worry about making the curves exact, as you can always return and edit them later using the anchor point tool. To fill your path with color, make sure to close the shape by returning to your original starting anchor point.
Examples in popular applications
The icon for the pen tool usually resembles a skeuomorphic based on a traditional ink or calligraphy pen.
Examples of vector graphics editors that feature a pen tool are Adobe Illustrator and Inkscape.
Some raster graphics editing software, such as Adobe Photoshop and GIMP (GNU image manipulation program), feature a pen tool similar to a full-fledged vector graphics editor.
