Shape extension

In the X Window System, the X Nonrectangular Window Shape Extension allows windows to be given arbitrary, non-rectangular shapes.

Two well-known applets that use the shape extension are oclock, which is a simple round analog clock, and xeyes, which shows two googly eyes that follow the cursor on the screen as if they were watching it. Most X systems have one or the other (or both) in their standard installations. In addition, some window managers use the shape extension to draw non-rectangular icons.