In 1963, Douglas Engelbart of the Stanford Research Institute invented the computer mouse, a handheld pointing device for computers. One of several experimental pointing devices developed by Engelbart for his ON-Line System (NLS), it was also called the "bug," but "mouse" became more popular because the cord on early models resembled a rodent's tail. An improved version of the mouse was developed by Bill English at Xerox PARC during the early 1970s which replaced the external wheels of the original model with a single ball, resembling an inverted trackball, that could rotate in any direction. Even the popular trackball, however, was done away with in the optical mouse, a device which utilized an optical sensor to detect movement. In 2004, Logitech introduced the first laser mouse, claiming a 20x increase in accuracy compared to the conventional optical mouse.