I have received so many questions about tuning an instrument from students and friends, and from beginners to pros that I have created a podcast on tuning and intonation.
You can listen to the podcast, or download it, from my music site: http://davewalkermusic.com/
I don’t want to duplicate the information from the podcast here, but I will give you a bit of background.
There have been a number of systems of tuning (or more precisely “intonation”) for Western music over the centuries. The ancient Greeks knew that sound was created by sound waves and that the regular pulsing of sound waves produces musical notes. Pythagoras is often credited with discovering that the interval of an octave (e.g. from C to the C above it) is an exact multiple, meaning that if a note vibrates at 100 cycles per second, the octave above it will vibrate at 200 cps, the octave above that at 400 cps, then 800 cps, and so on the same way.
The next important interval is the fifth, which is simply five steps up the scale, for example from C to G (C-D-E-F-G). Pythagoras is said to have discovered that this is the next simplest ratio, 3/2 which means that if the lower note vibrates at 100 cps the fifth above it will vibrate at 150 cps (100 x 3 / 2).
The podcast carries on from there, but in case you are unfamiliar with the jargon, our major and minor chords are called triads because they are made up of three notes, a root note and the note a third above it and the note a fifth above the root. For example, a C major chord consists of C, E, and G. E is the third (count C-D-E) and G is the fifth (C-D-E-F-G).
Our system is called equal temperament, and is based on a sequence of fifths so that if you continually move up by fifths you will return to your starting point. Starting at C this would be:
C-G-D-A-E-B-F#-Db-Ab-Eb-Bb-F-C
It works the same way if you descend by fifths (just the reverse of the above):
C-F-Bb-Eb-Ab-Db-F#-B-E-A-D-G-C
Here it is in a diagram that includes the key signature. You will see that at the bottom we change from flats to sharps (the keys of F# and Gb have the same notes, but they are spelled differently):

The diagram looks very neat but we have had to make some compromises for it to work out this way, and it is these compromises that make tuning more difficult than it seems to be, such as when you know that every string is in tune with the one beside it but a chord sounds out of tune. Listen to the podcast to find out why!
This podcast is a test. If there is enough interest I will do more of them.