AOL Instant Messenger is ending its two-decade run on Dec. 15.

Chances are, you stopped using it years ago. But reading those words, you might feel a nostalgic pang. AIM, as it is known, laid the foundation for the abrupt, instantaneous, chat-centric communication we now take for granted. It was also the coolest thing AOL ever did.

The late 1990s was a period of technological growing pains. Cellphones weren’t smart. People dialed up the internet through beige boxes with squawking 56K modems. The only glimmer of an always-on future came from shiny “America Online” discs that arrived in the mail. It wasn’t cool, but it was the Facebook of its time. Family and friends signed up, mostly because loved ones were there.

One AOL feature did induce envy: a window where you could type anything you wanted, and your “buddy” could reply immediately. This was instant messaging. AOL may not be the inventor—it actually acquired IM pioneer ICQ in 1998—but by adopting it, the company changed the world.

Email was too much like regular mail: You needed a purpose for writing. You started each missive with “Dear So and So” and closed with “Sincerely Yours, etc. etc.” Clicking Send meant, eventually, contacts would receive your notes—later on, that is, when they sat down to check their email.