The X.500 standards defined a universal directory/address books service. They were published between 1984 and 1988, alongside the X.400 standards.
Much like X.400, the standard didn't achieve mainstream acceptance as the more straightforward internet-based standards achieved ubiquity.--Nigel Dutt
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While it is correct to say that X.500 was developed concurrently with X.400, the work began during the 1980-1984 study period.
Thanks Debbie, I’ll fix the text to make it clear that I the dates refer to publication of the standards–please let me know if you find anything else that should be fixed.–David Ferris
CCITT published the first set of X.500 Recommendations in 1988.
Although implemention of the X.500 Directory Series in general was scarce, the X.509 became one of the most commonly implemented features on the Internet, also known as the https://