Lets get back to the basics again. This time the topic is about the layout of the guitar -

What exactly are the positions of guitar chords (position on the guitar neck) : Nut, 3rd, 4th, 5th, 7th, 9th, 12th, etc.?

This diagram from one of my earliest posts (design of the guitar) showed this but I realized that the verbal description was absent there.

The positions on the guitar neck refer to the fret farthest towards the head, from which any of the strings played in the chord, is vibrating. The positions are thus numbered from the head towards the body, just like the frets. The concept will be clearer with the following examples.

The chord positions of the C major chord are shown below. The chords are:

C major at the nut / open position : 032010
C major at the 3rd position : 335553
C major at the 8th position : 8-10-10-9-8-8


This principle is the same for all chords - major or minor.

Thus we note that:

• The position of the chord states where on the guitar neck it is supposed to be played
• The fret position determines the shape of the chord to be played
  
• The fret position of the bar in the bar chord is the position of the chord.
• The chord is in nut position when even one string is played as open
  
• The fret position of the finger closest to the head of the guitar is the position of a chord when it is neither played as a bar, not as an open stringed chord.
• When we write out the chord as fret numbers (E.g. C major - 032010) the smallest number in that series gives the fret position of that chord in that shape.

Other points to note:

• When we play a modified chord like a 7th or 9th, etc the same principle applies. Eg: the version of G#m7 - 355363 is in the 3rd position.
• When we play chords in non-standard ways - like if we play C major as a mixture of its nut and 3rd positions - say, 00503 - the chord will be in nut position and not the 3rd; for similar reasons B5 - 004402 - is in open position, while B major - 224442 - is in the 2nd position

Use of different positions:

The further down the neck we go, the shriller the notes and chords get. Though the chord is the same, the frequencies of the pitches of the constituent notes goes on rising. Thus the nature of the sound varies from one position to another for any given chord. These variations may be used at different positions in a piece to create a "change without a change" sort of feeling.
http://basicmusictheory.blogspot.com/2009/08/positions-of-guitar-chords-nut-3rd-4th.html