What needs to be known before you can say “single or double” is what the beginning canvas (your current hair) is and what your hair goal is. If your hair is virgin (has not been chemically processed at all), achieving a variety of outcomes can be a single process, where previously colored hair can get a little tricky and require more than one process (but not always). If you’re thinking about a drastic change, that usually calls for a double process. Here are some examples of single processes: