There are quite a number of traditional melodies which make use of the same harmonic conventions as other forms of music such as jazz. As far as I'm concerned, anything which contributes to an understanding of the harmonic structure behind such music on this forum is to be welcomed, and I see no problem with Josepp or anyone else discussing it in a serious manner.

The only area where I would comment on Josepp's diagram drawing is that, after time, a diagrammatic representation of the harmonic progression as shown by the circle of 5ths won't be necessary. Experience of playing through chord sequences - particularly jazz and some blues sequences and sequences in traditional tunes - will reveal the underlying patterns both to the ear and to the instrument. Once the patterns are understood - and I personally think in patterns on the guitar - then matters like transposition become less mechanical and more organic and natural.

Playing jazz with other musicians is a wonderful way to haul yourself up by the musical bootstraps. In my jazz days, quite often a guest musician would drop in for a blow. The etiquette was to ask him to choose a number to play - and pick the key. Occasionally, there would be a number that I hadn't heard before; occasionally the musician would prefer a key that wasn't the norm... What better way to get to grips with the whole business of transposing quickly into a different key and/or getting to grips with an unfamiliar chord sequence?

This is where some early knowledge of the harmonic progression underlying the circle of 5th was useful to me. I very quickly discarded the diagram and just 'knew' the patterns in it.