X509 is the name of the standard which defines Public Key Infrastructure and the format of SSL/TLS Certificates. My question is: What does x509 mean?

2 Answers 2


X.509 is a standard number/index defined by ITU-T, telecommunication and standartization commitee.

X stands for Data networks, open system communications and security standards track and every standard has a number. X.500-X.599 standards define directory services (such as DAP, LDAP, etc.). And 509 is just a number of standard in X track.

  • Excellent! Thank you so much! -- Do you by chance know why certain numbers were skipped? (502-508, 512-517, etc)
    – Eddie
    Sep 25, 2020 at 19:12

According to Wikipedia:

X.509 was initially issued on July 3, 1988 and was begun in association with the X.500 standard.

X.500 is a series of computer networking standards covering electronic directory services. The X.500 series was developed by the Telecommunication Standardization Sector of the International Telecommunications Union (ITU-T). ITU-T was formerly known as the Consultative Committee for International Telephony and Telegraphy (CCITT).

X.500 was approved first in 1988.[1] The directory services were developed to support requirements of X.400 electronic mail exchange and name lookup. The International Organization for Standardization (ISO) was a partner in developing the standards, incorporating them into the Open Systems Interconnection suite of protocols. ISO/IEC 9594 is the corresponding ISO identification.

X.400 is a suite of ITU-T Recommendations that defines the ITU-T Message Handling System (MHS).

I could not find why X.400 is named X.400 though, also why X509 and not X.502.

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