In the context of programming, style can refer to many things. Foremost, it refers to the format of the source code: how to use indentation, when to add comments, how to name variables, etc. It can also refer to many other issues, such code organization, modularity, and efficiency. Of course, stylistic concerns are often matters of individual taste, but often validity and portability of the code will ultimately depend on stylistic decisions made in the process of code development. Hence some stylistic choices are viewed as universally bad (e.g. not prototyping every function just because ``the code compiles and runs fine as is'', etc.). Admittedly, it is easy to not have any style, but it takes years to learn what makes a good one. A good programming style can reduce debugging and maintenance times dramatically. For a large package such as PSI3, it is very important to adopt a style which makes the code easy to understand and modify by others. This section will give a few brief pointers on what we consider to be a good style in programming.