Thank you very much, redheffer. Your note was very helpful to me. You're right about studying harmony. But the problem is that I don't have time. Another problem is that it's hard to find a good music teacher. The first teacher for me was a friend of mine, but he had limits. He told me how to play chords, but he lacked the basics of harmony. So, I turned to the Internet and found it a great source, as long as there are people like you out there. But taking a lesson is the best choice, so I'll try to find a good teacher. Thank you again and expect some silly questions for me the next days :-)