As you may have already guessed, ``joints.c'' contains a program written in C.
On our computer systems, the file names of C programs end with the characters ``.c'' (a period and a lower-case `c'). This convention allows you to just look at a file name and know what language the program was written in.
Many programming tools, including Emacs, know about this convention so that they too can tell what language a program was written in. For this reason it's important for you to follow this naming rule.
Now look at our example program again. Read the English text at the top of the program and you'll discover that it solves a kinematics problem. ( Kinematics is the study of motion--position, velocity, acceleration--without regard to the forces that cause motion.) We'll discuss the specific problem that this program solves shortly.