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Prodigy was a big early Internet service provider, and of course by the late nineties, the most popular Internet protocol was the web; most users spent most of their Internet time in a web browser.

But if you go back a few years before that, say before 1986, before the web existed, that was even the era predating MacOS 5 and Windows 3.x, when people hardly expected a graphic user interface.

How did Prodigy provide GUI for its subscribers at that time? X-protocol was bandwidth-hungry by 80's standard even via LAN or with compressing extentions, let alone through Dial-up modems under 2400bps.

Related: What did AOL use for pre-web GUI client?

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Prodigy used NAPLPS (North American Presentation Level Protocol Syntax) to encode its graphical displays. This is a vector-based graphics originally intended for videotex and teletext services (in North America). Client-side behaviour was implemented using proprietary programming languages.

Prodigy clients were available on at least DOS, Windows 3, and classic Mac OS.

You can see screenshots of Prodigy, including games, in Benj Endwards’ articles on Prodigy’s 20th anniversary and on the MadMaze Prodigy game, although static screenshots provided a limited representation — Prodigy screens were slow to load on modems of the time. You can relive that partially by playing MadMaze-II on a slow Internet connection. If you’re familiar with teletext, the screenshots from the first article should ring a bell (albeit in higher resolution):

Prodigy on December 25, 1992

Benj Edwards (him again!) wrote a detailed article explaining how Prodify works for The Atlantic. Michael Dillon wrote a description of the language itself.

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