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I've run across the idea of "trading fours" when playing in (and listening to) jazz groups; i.e. where two or more musicians take it in turns to play four bar licks or solos. I understand that it can encourage creative interaction between musicians.

However, I was wondering why it seems to be specifically four bars? Would trading twos or eights (for example) be less effective?

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up vote 8 down vote accepted

It's just a name: it used to be based on four bars, which probably would comprise one set of chord changes (eg doowop, I vi IV V), but could just as easily be two or eight bars. It's like calling a song's bridge a 'middle eight', even though the number of bars may be different. The Beatles always called their bridges 'middle eights'.

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In swing setups such (e.g. tenor sax battles), it is not uncommon so see "trades" of varying (typically decreasing) length : trade 16, then trade 8, trade 4 and sometimes even trade 2 then trade 1, each time building up the tension. Things could also end in both musicians improvising simultaneously. Nice example from Robert Altman's Kansas City:

As for the reason behind 4 bars, I think it comes from the structure of the typical 32 bar form AABA, in which the 8 bars of A are often divided in 4 bars (tension / question) often ending in half cadence (V7) + four bars (relief / answer) ending in IMaj7. This means having 4 bar solo by one musician followed by for bars by another is quite natural.

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"Trading twos" is definitely a phrase people use. "Trading eights" might be.

More generally, you can call it "trading licks".

8 bars is long enough to be thought of as a whole solo. Get any longer than this and you're not really "trading" any more; you're just taking it in turns to solo.

When I say "trading", I mean that you base your lick on whatever the player before you played.

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Generally it'll be even numbers, 2, 4 , 8 as a 'line' of a song tends to be that long, so phrasing sounds more balanced. It follows from songs usually being 8, 12, 16 or occasionally 24 bars long. Never heard in a band "Let's trade 7s." Although that could be interesting. Both to play and listen to...

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In most tunes, four bars is the smallest unit you will find a complete phrase, musical idea, or melody. although you will often see it in eight as well (a complete chorus or bridge or A section of the tune), but youll never see it in two because two bars is too short to express a complete musical idea.

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