What is the 't' folder?

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http://unix.stackexchange.com – Sorry in advance, if this sounds like a stupid question, or something basic/trivial... I'm mainly a windows guy, programming C#, but i often do use technologies that were intended for linux machines like git, MySQL, perl scripts, memcached, php, etc... And therefore i am exposed to these tools. I like looking at the code base of these tools every once in a while, and something i realized in many code bases is a folder called 't' with a bunch of files with the 't' extension. What are these files ? How come the folder doesn't have a more descriptive name ? (HowTos)