In my first discussion of the differences between playing the guitar and clarinet my conclusions seemed to be that in many ways the guitar was physically easier to play but more difficult to produce coherent improvisations simply because it was easier to play, for example semitone modulations in a tune could be accommodated with no more consideration than sliding up or down a fret. Some further time playing the clarinet has made me revise this assessment to conclude that it is more difficult to play inventive and well-structured improvisations on the guitar simply because it is more difficult to play.
The reason for this difficulty is the duplication of notes on the guitar fretboard. On a clarinet if I want to play a middle C there is only one possible fingering so there is an indivisible link between the sound of middle C and that fingering, and this can be reinforced by many different practice techniques. If I had perfect pitch (which I don’t have by a distance of many light years) then with some practice I could play any note on the clarinet that I heard without having to worry about the fingerings (there are a few alternative fingerings on the bottom keys but we can ignore that for the moment).
On a guitar there are many ways to play a middle C. 1st fret on the B string, 5th fret on the G string, 10th fret on the D string and 15th fret on the A string. Each of those notes could be fretted using one of four left hand fingers giving a total of 16 different fingerings! Mick Goodrick, in his thoughtful book The Advancing Guitarist, has likened playing the guitar to playing 6 chromatic keyboards offset to each other by intervals of a fifth except of course between the G and B string when it is a fourth. This makes it very difficult to establish any clear link between the pitch of a note and its fingering on the guitar and may account for much of the mind-numbingly boring jazz guitar that is played using a limited repertoire of licks in fixed patterns based on strict positional (i.e. across the fretboard rather than up and down it) playing.
Interestingly the techniques used in gypsy jazz guitar seem to get round this problem by using a limited range of strings and fingerings because the techniques are much more based on movement up and down the fretboard rather than across it. We know that Django could only use two of his left hand fingers and most current players seem to use only three for the vast majority of time so that immediately reduces the fingering possibilities by at least 25%. On the acoustic gypsy jazz guitar many notes, though theoretically possible such as a middle C on the 15th fret of the A string, are not practical either through lack of access to the fretboard or more likely insufficient volume to cut through the sound of the rhythm guitar. This has lead to techniques with big movements up and down the fretboard using usually the highest available string for the note required which gives far more chance of establishing a firm relationship between that note and its fingering. Perhaps this is why much gypsy jazz guitar sounds so much more structured and purposeful than many fully electric jazz guitar recordings?
One area where the guitar holds a massive advantage over the clarinet, or any other wind instrument, is the relationship between relative pitch of notes and their fingering. On the guitar I know that if I play one note on any string and another note two frets higher on the string above (with the exception of the G/B string combination) then those notes will produce an interval of a perfect fifth. On the clarinet there is no clear relationship between fingering and interval, especially when moving between the upper and lower registers which overblows by a twelfth rather than the octave that saxophones and flutes do. I am not sure of guitarists can take full advantage of this property when playing at fast tempos but when listening to gypsy jazz guitarists playing ballads you can often hear them ‘stretching’ for a note which they know is there to be played but which lies outside any usual pattern that they use. A great example of this is the beginning of Lolo Meier’s solo on ‘I surrender dear’ where he takes a little motif and shifts the second note of the repetition of the motif up to accommodate the underlying chord change and to construct a very melodic phrase.