Forum: music.stackexchange.com
Tags: <jazz><improvisation><band><jamming>
Title: Why do jamming musicians "trade fours" (rather than any other number)?
Question: 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?

Answers:
sents: text: 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.
text: It's like calling a song's bridge a 'middle eight', even though the number of bars may be different.
text: The Beatles always called their bridges 'middle eights'.
answer_details: author: music.stackexchange.com/users/8587/No'am Newman
score: 9
sents: text: In swing setups such (e.g. tenor sax battles), it is not uncommon so see "trades" of varying (typically decreasing)
text: length : trade 16, then trade 8, trade 4 and sometimes even trade 2 then
text: trade 1, each time building up the tension.
text: Things could also end in both musicians improvising simultaneously.
text: 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.
text: This means having 4 bar solo by one musician followed by for bars by another is quite natural.
answer_details: author: music.stackexchange.com/users/1079/gurney alex
score: 9
sents: text: "Trading twos" is definitely a phrase people use. "Trading eights" might be.
text: More generally, you can call it "trading licks".
text: 8 bars is long enough to be thought of as a whole solo.
text: Get any longer than this and you're not really "trading" any more; you're just taking it in turns to solo.
text: When I say "trading", I mean that you base your lick on whatever the player before you played.
answer_details: author: music.stackexchange.com/users/1252/slim
score: 6
sents: text: 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.
text: It follows from songs usually being 8, 12, 16 or occasionally 24 bars long.
text: Never heard in a band "Let's trade 7s."
text: Although that could be interesting.
text: Both to play and listen to...
answer_details: author: music.stackexchange.com/users/2808/Tim
score: 2
sents: text: There are great answers already here, but perhaps the most important consideration is making sure no one gets cut short when the trading is over.
text: Consider a 12-bar blues.
text: It wouldn't really work for the soloists to trade off every 8 bars, because whoever goes second in soloing would only be halfway through his/her turn when the 12-bar form ended.
text: (In particular, the first soloist would take bars 1-8, and the second soloist would take bars 9-12 followed by bars 1-4.)
text: Spilling over like
text: that can undermine the listener's sense of the form itself.
text: Our ears generally expect to hear the soloist change at the very top/beginning of the form.
text: If that doesn't happen, it feels uneasy, and we might get the feeling that we don't actually know where the top of the form is.
text: Jazz songs very frequently use 12-bar forms, 16-bar forms, or 32-bar forms.
text: Letting the soloists trade off every 4 bars is great because all of those forms are evenly divisible by 4.
text: Thus no soloist's turn will be interrupted by the end of the form.
text: Trading 2 bars at a time is also common, and this frequently comes near the end of the trading, after having spent some time trading 4 bars.
text: Trading 8 bars is also pretty common, although only if the entire song has an integer multiple of 8 bars.
answer_details: author: music.stackexchange.com/users/40842/jdjazz
score: 2
sents: text: In most tunes, four bars is the smallest unit you will find a complete phrase, musical idea, or melody.
text: Although you will often see a complete chorus or bridge or A section of the tune in eight as well, but youll never see it in two because two bars is too short to express a complete musical idea.
answer_details: author: music.stackexchange.com/users/5919/Michael Martinez
score: 1

Summary of the answers:
Trading Fours is 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 follows from songs usually being 8, 12, 16 or occasionally 24  bars long. Trading 2 bars at a time is also common, and this frequently comes near the end of the trading, after having spent some time trading 4 bars. Trading 8 bars is also pretty common, although only if the entire song has an integer multiple of 8 bars.
In reality, this could be just a musical term and the actual trade may not be strictly four bars. However, the term "trading fours" originates from the fact that four bars or a multple of four is normally the length of a complete phrase or melody. "Trading eights or "trading twos" is certainly possible, but "trading twos" is less likely since it's quite a short length of time to express a complete musical idea.