Hey Friend,
Let's do this.
In the beginning, there was a void... a nothingness.
Out of the nothingness came a sound, and the Universe listened. Then multiple sounds found their way out of the void... The sounds worked well together and the key of C major was born.
Individual notes rang out, sounding beautiful in and of themselves, but then something happened: two notes sounded together and made harmony. It was a random occurrence, but the Universe was pleased. It sought to duplicate this beautiful pairing and found that first sound combined with another pleasing tone created a huge sound.
And so the Power Chord blew the Universe's mind. Combining that root note with the fifth scale note produced enough sound that Rock adopted the power chord as its patron saint.
But the Universe wasn't done with its cosmic dance. It combine more notes until it came up with the triad - a combination of notes that would sway the hearts and minds of mortals. The Universe found so many varieties of sound that it was satisfied with the twelve little notes that came out of the nothingness...
The Universe presented its twelve little notes to humankind, and we have made music with them ever since.
Yeah, it mighta happened like that... or not.
Like any other key, C major has only seven notes. From those seven notes we can create countless chords, and a model for every other key (and chord).
And if we're to start with C major, it makes sense to start with the C major chord.
A triad is a chord that has three notes. "Tri" = three. You can classify any three-note chord as a triad. Major, minor, suspended 2, suspended 4, diminished, augmented - all triads. Even though you could strum up to all six strings when playing one of these chords, they are still triads.
For example, when a chord such as an open C major is played across all six strings, yes, you're strumming each string, but the notes are duplicated. The open E string on either side of the neck give us one note of the chord. The third fret of the fifth string and second fret of the B string each provide the root note of a C major chord with the C note.
The open third string (G) gives us one lonely G note, but it's enough to round out the C major chord.
When then notes are doubled up in the chord, it has the effect of making the chord sound bigger, bolder, fuller. This is actually a technique used quite a bit in the recording studio. It's not uncommon to have multiple tracks with the same instrument part layered on top of each other, again to produce a fuller, bolder sound.
Getting back to open C major, you could strum just the fifth, fourth and third strings to produce the C chord. But compare that to the doubled notes when you strum all the way through, and you can hear the difference in the fullness of the sound.
Examples to follow, but for now I have to get off to work...
Peace~
Dave | 
