If you’re a Gmail user with keyboard shortcuts enabled, you’ve probably noticed that you can use the j and k keys to navigate through your inbox. Hitting j takes you to an older message, while k moves you to a newer one. This navigation style isn’t unique to Gmail—it’s incredibly popular across the web. You’ll find it on sites like MakeUseOf, Google Reader (RIP), Tumblr, and countless developer-focused applications.

But why specifically J and K? Why not other keys?

The history behind this is actually fascinating. Let’s take a look.

It all started back in the UNIX era. When Bill Joy created the famous vi text editor for Unix, he was coding on an ADM-3A terminal. The catch? The keyboard on that terminal didn’t have dedicated arrow keys! You can actually see the layout of the ADM-3A keyboard in the illustration above.

Instead of arrows, the keys h, j, k, and l had tiny arrow symbols printed right on them. By pressing these keys—often in combination with the Ctrl key—users could move the cursor around the screen.

Times have obviously changed. Modern keyboards have dedicated arrow clusters, and layouts have evolved. But those original j and k keys became so ingrained in the muscle memory of developers worldwide that they stuck around. UNIX became one of the most influential operating systems in history, and its developers carried the h j k l navigation scheme into the modern web and desktop applications we use today.

If you’re curious, Wikipedia has some great, detailed history on the vi editor and the HJKL keys.