CHILL, short for CCITT High Level Language, is a stalwart of the telecom world. Created in 1980 by CCITT (now ITU) to program telephone exchanges and other communication systems, imagine a language as robust as Ada but focused on one niche: real-time reliability and systems that must run for years without fail.

The language is statically and strongly typed, with a structure that makes maintenance and readability straightforward. There’s even an object-oriented variant, Object CHILL, for those wanting more modern constructs. While CHILL is no longer in the spotlight, it still runs in legacy systems at companies like Siemens and Alcatel - and in some railway signalling systems.

Want to dig deeper? There is a CHILL compiler from ITU and a free implementation was present in GCC up to version 2.95. It’s not the language you’d pick for a new web app, but if you enjoy exploring computing history and the languages that built today’s systems, CHILL is a fascinating trip back in time.