The software industry is full of patterns and design guidelines. As you continue coding and creating different applications, you will discover their importance and understand when to choose one technique over another. In the preface of the book, I told you about the Pareto principle, or 80-20 rule, which states that 80 percent of outcomes come from 20 percent of all causes. This is why, in this part, I show you the techniques that are commonly used to build real-world applications. Once you master these techniques, you will get a hint about the answer to the following question: what does a professional programmer think before writing a piece of code?