plucking particular strings, you're almost exclusively
playing chords. But what's a chord? A chord is a set
of notes (three or more) which are sounded
simultaneously.
The most basic chord is known as the tonic chord i.e if
the chord is a C chord, then the tonic note is C. The
other notes will be built on that at intervals of a third
above. The musical scale is C D E F G A B C - this
means that the note a third up will be E and the third
above that is G. The whole chord is C E G. This
formula is the basis to working out the notes for any
chord.
The notes in a chord do not have to played in the scale
order though, they can be played E G C - moving the
tonic note of C to the next octave makes this a first
inversion chord i.e. the chord has been inverted and the
first (lowest) note is now the highest adding a different
tone to the overall sound.
If you invert the chord again to make it a second
inversion i.e. G C E, then the tone of the chord is
different again even though you are using the same
notes just in a different order.
A common chord used in guitar music is the 'seven
chord' - the chord is written as C7. This is an exception
to the rule of the tones being worked out to be a third
above each other. The tonic note in our example is
still C, the second tone is a third above (E), the third
note is G. The fourth note is a seventh up from the C
which is B.
The whole chord would be C E G B.
One of the classic ways to get a discordant sound is to
play a seventh interval, which is the tonic C and the
seventh B. The way to fix this is to leave out the C
altogether and make the chord only E G B.
Now you can go and work out your own chords for your
songs. Don't forget about sharps and flats when you're
working out the chord notes!