In order to write music correctly and safely, it is necessary to respect the well-defined universal rules. The great composers of the past have left the world an immense musical heritage, and thanks to their works we have learned the rule of composition. Rigid rules, ancient: but perfect and safe. The musical composition is a very fascinating discipline that allows us to enter the world of invention and inspiration. The musical writing gives us the possibility to put on the score a particular emotion, to describe an image, etc. All this is possible thanks to the rules of the harmony and composition. Harmonizing a melody, a given bass or a given song, concatenating or revolving an agreement, know how to interpret with the writing a figured bass, a melody that suddenly passes through the mind or that we are whistling, etc. The learning of the composition is fundamental for every musician and even more important is the rule of composition. The study of this marvelous discipline makes us participate with writing, at the realization of our desires for a musical instrument or for a symphonic orchestra.