From Rosetta Code
This programming language may be used to instruct a computer to perform a task.
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Listed below are all of the tasks on Rosetta Code which have been solved using PlainTeX.
Your Help Needed
If you know PlainTeX, please write code for some of the tasks not implemented in PlainTeX.

Plain TeX is the name used to refer to the default set of macros which are always present on all TeX systems (and it is the default preloaded format for tex executable). They were written by Knuth himself, and provide the basic to start using TeX (since "raw" TeX is not ready-to-use for documents creation). Often when one says simply "TeX document", s/he means s/he is using the Plain TeX macros, even though nowadays the success of the complex feature-full LaTeX macros package creates a little bit of confusion and a lot of people, ignoring that LaTeX is just one of possible macro packages for TeX, think that LaTeX is TeX and viceversa.

Extension to the Plain TeX exists, like eplain(TeX).

Even though LaTeX is nowadays almost a typesetting "standard" when using a TeX system, Plain TeX is still usable and used.