Watch the Adding Safe Extensions online guitar lesson by TrueFire from Play Acoustic Guitar 8: Advanced Techniques
Adding Safe Extensions - Harmony Fundamentals: 3 is a video guitar lesson presented by Massimo Varini and is sourced from Pop Guitar Survival Guide.
Before adding extensions it is important to think that this is a process that is often useful and even essential, but I would say there is nothing written indelibly in the rules. Sometimes playing chords in their simplicity of the triads is the best solution for the song you're going to play. There is not a competition to add more extensions and make arrangements more complex: the important thing is to communicate the message that the song in it wants to take. So what I show in these lessons are possibilities and tools to get different sounds - it is up to personal taste using these colors. In this case, we will see the extensions that have less impact harmonicly, safe extensions that perhaps in the chart of the chords are not written but that can be added to get a deeper sound. It's also important to think about the mixing volume of the guitar in question - if a guitar has to be very low in the mix and make a rhythm in the background it could be much better to choose not to put in extensions or add only the safest.