Which tool is commonly used for creating masks in After Effects?

The Pen Tool is commonly used for creating masks in Adobe After Effects due to its precision and versatility. This tool allows users to draw custom shapes directly onto layers, defining specific areas of the layer that can be hidden or revealed. When you use the Pen Tool, you can create complex, freeform shapes that can follow any path, making it ideal for detailed masking work.

In addition to drawing shapes, the Pen Tool also enables you to manipulate the anchor points and handle curves, providing further control over the mask's appearance and behavior. This is particularly useful when you need to create masks that require intricate outlines or need to conform to specific features in your footage.

While other tools mentioned have their own purposes—like the Brush Tool, which is used for painting and texture effects, the Rectangle Tool, which creates rectangular shapes, and the Shape Layer Tool for creating vector graphics—none offer the same level of detail and manipulative capabilities when it comes to mask creation as the Pen Tool does. Therefore, it stands out as the primary choice for artists and editors looking to create tailored masks in After Effects.