The first computer mouse, patented in 1970, had no rolling ball inside—it moved on two metal wheels set at right angles.

The first patented computer mouse rolled on two perpendicular metal wheels, not the ball most people imagine—that came a few years later.

The device you picture when you think "mouse"—a smooth ball rolling underneath, spinning tiny rollers—is not what Douglas Engelbart patented. His original design, filed in 1967 and issued as U.S. Patent 3,541,541 on November 17, 1970, moved on two hard wheels. The ball came later, from a different engineer, at a different company.

Read the patent itself and the mechanism is spelled out plainly. Engelbart's "X-Y Position Indicator for a Display System" describes "a small housing... having two wheels." The key line: "The two wheels are mounted with their axes perpendicular to each other and each wheel is attached to a potentiometer." One wheel tracked motion east-west, the other north-south. Drag the housing across a desk and each wheel turned only as far as its own direction of travel allowed—the perpendicular arrangement let the two potentiometers report an X value and a Y value independently.

Why the first mouse used wheels instead of a ball

A potentiometer is essentially a dial that changes electrical resistance as it turns, the same component behind an old volume knob. By wiring each wheel to one, Engelbart converted physical rolling into a measurable voltage the display system could read as position. The wheels weren't a limitation he overlooked; they were the whole point—a cheap, direct way to translate hand motion into two numbers.

There was an awkward side effect. Because each wheel gripped the surface, moving diagonally forced one wheel to skid rather than roll cleanly, which threw off tracking. The design worked well enough for Engelbart to demo it publicly, but the geometry left an obvious problem for someone to solve.

The physical prototype was famously humble. Bill English, the engineer in Engelbart's lab at the Stanford Research Institute, built the first working unit by hand—a carved wooden shell with the two wheels poking out the bottom and a cord trailing from one end. It is the block of pine, not a sleek plastic shell, that sits behind the whole story.

How the rolling ball actually arrived

The version everyone remembers came from the same man who built the wooden original, but only after he changed jobs. According to the Computer History Museum's account in its "Revolution" exhibit, Bill English left SRI and moved to Xerox PARC in 1971, where his group developed PARC's own mouse—this time a rolling-ball design, in the same family as the 1968 Telefunken mouse built in Germany. SRI International, in its "75 Years of Innovation" history, dates the switch precisely: "In 1972, Bill English, now working for Xerox PARC, replaced the wheels with a ball to create the Ball Mouse."

The ball fixed the diagonal problem. A single sphere could roll freely in any direction, spinning two internal rollers set at right angles—the same X-and-Y logic as Engelbart's wheels, but with the geometry moved inside where nothing had to skid. That mechanism, refined over the next two decades, is what shipped inside desktops well into the 2000s, until optical sensors retired the ball entirely.

So the invention most people credit to a single 1970 patent is really two acts by two arrangements. Engelbart's patent established the idea—a hand-held housing that reports its own X-Y position to a screen. English's later work at PARC gave it the moving part everyone can name.

One thing still worth arguing about is who "invented the mouse," and the honest answer resists a clean headline. Engelbart holds the patent and conceived the function. English built the first prototype and then, at a rival lab, replaced its core mechanism with the design that actually spread. And Telefunken's ball mouse in Germany appeared around the same time, complicating any tidy line of descent. The patent settles what the first mouse was made of. It doesn't settle whose name belongs on the thing in your hand.