Ever have a problem on your Mac, where you can’t type the letters and numbers 7, 8, 9, U, I, O, J, K, L, and M—and maybe some others?

It’s probably Mouse Keys at play, an accessibility feature in macOS that mimics an old IBM-style keypad feature.

Num Lock and its computer history

Keyboards still have vestiges of their origins on typewriters and mechanical calculators. We mostly ignore those in our day-to-day work. The Shift key, originally meant to literally raise a set of typebars in a typewriter, merely shifts among upper and lower case. And you know better than to press Caps Lock when entering passwords—Apple even alerts you when you have it accidentally enabled.
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Early 84-key IBM keyboards had a numeric keypad that doubled as arrow and other keys for text-screen-based interactions.

The Num Lock key was a product of the intermediate age, where mechanical and digital met. Some IBM computer keyboards didn’t have separate arrow keys, but they did have a numeric keypad for fast number entry. IBM doubled up: It added a Num Lock key that flipped the numeric keypad to act as arrow keys to move a cursor around a screen-based interface, before computer mouses.

On certain laptops, including older Macs, Num Lock had a different function. Lacking a numeric keypad, Num Lock turned part of the main keyboard into a pseudo-keypad. (There was sometimes a unique Num Lock key, sometimes it doubled with a Clear key and Shift, and sometimes it was a function to invoke, like through F6 on a Mac.)

The pseudo-keypad layout relied on sets of four keys starting with 7, 8, 9, 0 and going down three rows to M. With this mode invoked, you could still type 7, 8, 9, and 0 and have those characters appear, as they’re still mapped to the same position. But U, I, O, P, J, K, l, semicolon, M, command, period, slash take on keypad functions.
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The PowerBook and some early MacBooks had numeric keypad symbols overlaying part of the main keyboard.

Apple stopped supporting Num Lock via F6 quietly back in 2008. It shouldn’t be possible to invoke this mode accidentally, as it shouldn’t exist. It’s available in Boot Camp with some laptops, although Apple doesn’t specify which.

An independent developer did create a Num Lock app—called simply NumLock—that would invoke the feature within Mac OS X; it hasn’t been updated in a few years, and I haven’t tested it in Mojave.