The acoustic guitars is the classic stretched string instrument as it has all the positions where the sound varies. These are the main positions that can be used to record an acoustic guitar.
Lets look at the different positions.
This is the standard crossed pair stereo miking position. This gives an overall sound of a guitar, it is not a tight presence sound like position B but has the combination of the striking sound, the bridge sound and the body sound. The stereo effect is not very wide but if you want the ambience of the room (like in a full strum rhythm track) it can be appropriate. This can also be a position for a single mike.
Position B is the most popular mike position and the one I recommend for normal acoustic guitar recording. The mike is placed about 15cm(6") from the guitar pointing at the end of the finger board but not directly at the sound hole. The pick sound is emphasised in this position giving a nice clean attack to the guitar. This position also has the mike pointing away from the fretboard so finger noise is reduced.
This mike is aimed directly at the bridge and is close around 10cm(4"). The sound here is harder sounding as it has less low frequencies and the mid range sounds are emphasised. If I wish to record a stereo guitar I usually use positions B and C and pan one mike left and the other right. In these positions the stereo spread is emphasised because
Position D can be used as an alternative to position C for a warmer stereo sound as it doesn't have the hardness of the bridge sound yet emphasises the warmer body sound. You must note that the mike position at the rear of the guitar causes the mike to be 180 degrees out of phase to the mikes in the other positions therefore a phase reversal must be used.
Why not use all positions? If you are about to record acoustic guitar tracks why not set up mikes in all positions and play with the balance of each mike to gain the benefits of each. You might have a great stereo spread between positions B and C yet adding some of position A will add fullness and body, or adding position D panned centre to do the same. Play around, don't just limit yourself to one position only.
How we wish the guitar to sound in the track determines how we track it to tape and how many tracks we use. Lets look at the various ways acoustic guitars are used.
Solo guitar as in folk singer.
Here you can either use position B and have the track in mono or you can create a stereo track using position B and C. The thing about folk singers is that they sing and play at the same time!! so the guitar mikes are going to pickup the vocal as well therefore any EQ, Reverb etc. that you put on the guitar will also affect the vocal. To get the minimum spill of the vocal into the guitar mike I recommend you use position B and raise the mike so it points down at the guitar at about a 45 degree angle but still in the B position. This tends to put the vocalist off mike to the acoustic guitar mike. (You can also do the same with the vocal mike by having it pointing up at the singer and away from the guitar.) Another method I've seen is to place a soft covered sheet of cardboard or timber horizontally above the guitar that divides the spaces between the guitar and the vocal, but your guitarist must be able to play without seeing their hands!! but it does work.
If the singer is going to overdub the vocal later then you can afford to make more of the guitar sound by recording it in stereo but the singer must play the guitar track without singing.
Strummed Rhythm Guitar.
In this situation you may wish to have a single stereo/mono guitar track or you may wish to multitrack the acoustic. I often double track an acoustic rhythm guitar with one panned left and one right. Another good method is to get the guitarist to play one part through then to put on a capo and play the same chords but in a different position on the guitar. This expands the guitar sound and sounds really good. Some people call it "adding a high strung" You play the first part in say the standard C position and then play the part capoed up to the third fret but play it in the A position. You can go even further , as I have often, and record two tracks in the C position and then do two tracks in the higher capoed position. The effect is a wall of acoustic guitars. You can pan the two high strungs left and right and pan the low strungs half left and right.