Okay, so first you need to understand your key signatures... if you don't then you will never get the idea of consonance and dissonance. Anyway, so first we will go over the figured bass system of chord identification to help you understand what it is that i am talking about.
Have you ever looked at music and seen some roman numerals and wierd little sharps and flats underneath chord progressions? Well, that's the figured bass. Essentially it's purpose is to let you transpose the music into other keys and it lets you know where the piece is leading you. In this figured bass you will see the progressions made. For every key there is a set of roman numerals. I, II, III, IV, V, VI, VII, I - and they all have names, but i won't get into that, essentially all you need to know is that tonic is I for now...
Dissonance is what results when you play something that doesn't fit in with the progression, or it just doesn't follow a certain pattern. It can happen with your chord formations, such as throwing a Bb into an F major chord, or you can have it with the progressions of chords in your piece, such as using the III or VIIdim. in a piece that is excluding them. This results in a sound that is not pleasant by itself, but when writing music, it gives it an edge that is otherwise not there.
Consonance is what happens when you follow all the rules and formations that you are supposed to. A lot of times you will find that I, II, V, and VI are usually consonant, and the others usually dissonant (described earlier). This means that assuming you stay with consonant chord progressions, you will have a solid piece that will have a good start and a good end. It will seem really strong and the movements fluid, but there will not really be much climax or denoument to your piece. It will be like a flatlining patient in the ER to be blunt, dead.
So if i can't use all dissonant chords, and i can't use all consonant chords, what do i do?
Well, obviously you have to do some sort of mixture of them. It's really not as complicated as it sounds. Take Metallica with the song Enter Sandman. The opening power chord riff starts low on a good consonant chord, and progresses to some that are a little off, the high pitch, but as a resolution to the dissonance and the fifth skip it steps back within that skip and flows back to a nice consonant chord again. Therefore you have that cool sound of edginess that ends really well.
Resolution Of Dissonance
Every time you use disonance you need to resolve it. Not only does it make for good practice, but it makes for good pieces, and even better satisfaction with your piece. Any time you have dissonance, say you are in the key of A Major, and you use a C chord in first inversion, you are going to have a dissonant sound, so what you need to do is resolve that. Now how you do that is really up to you. You can skip right to one of the nearest consonances, or you can let the notes lead you there. If you were in a first inversion C, you would probably want to go to a solid E, so that you would fall on the V chord, making it strong.
If you don't understand anything about this article or have questions feel free to email me at Toastter@gmail.com.