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As a pianist who have only experienced classical music yet, I recently got my hands on The jazz theory book, because I am curious about this kind of music that is totally new to me.

The theory is focused on chords progressions and the scales that complements them, which led me to wonder a few things :

  • Why choose to describe music with chord progressions ?

I mean, it doesn't carry that much informations. The chords themselves can be played in a million possible ways (with inversions, arpeggios, rythm, voicing, dynamics, etc.).

I understand that different chords convey differents feelings or emotions, and can be descriptive of the particular progression of a piece but there is so much more to it !

  • How exactly do jazz pianists use chords progressions when playing a lead sheet ?

I would be surprised to ear that they just play chords with the left hand while playing the melody in the right hand, as indicated in the sheet music.

Is the lack of informations given in a lead sheet the reason why improvisation is such a big part of playing jazz ?

In other words, do jazz pianists have to improvise over every bar of their lead sheets, by deciding on the fly of the way they are going to play all the (important ?) stuff that is missing on the sheet ?

  • A lead sheet contains as much information as is needed - the chords, and the basic melody. without those, there won't be a tune and accompanying harmonies. So, reading a lead sheet, playing the chords provided, in the order written, will give pretty well all the info. needed. Players will play whatever voicings they feel will best fit at the time, and perhaps put occasional extra notes, mainly from those chords, under the right hand to notes played. – Tim Nov 5 '17 at 19:32
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As you've suggested, there's a lot of improvisation in jazz. Almost everything is open to interpretation by the musicians--including melodies, rhythms, feels, and specific voicings for each chord. For most genres of jazz, the chords are usually the most fixed/immutable component of a song. (That said, chords are not completely unchanging. Chord substitutions and playing outside both introduce on-the-fly changes to the chord progression.)

In addition, there is a rich tradition in jazz of learning by listening to famous recordings. This old tradition extends far into modern-day jazz. As a simple example, imagine you're on a jazz gig with some musicians you've never played with before. The saxophonist calls the tune Autumn Leaves. One of the first things you would ask before the group starts playing is, "Do you want to play the Miles intro?" Everyone would know this intro because it's such an iconic version of the tune.

Turning our attention back to lead sheets: you're on a gig and there are ~300 or more songs you might be asked to play. Only some of these songs have famous intros, feels, rhythms, etc. that you're expected to know from famous jazz recordings. From a publisher's perspective, it wouldn't make a lot of sense to write out all of those introductions along with the chord changes. What if a song has two different famous interpretations? What if, over time, musicians change which interpretation they favor? There is likely just too much subjectivity in picking and choosing particular arrangements to include in a book with lead sheets/chord changes. And doing so would abandon the original song's core, which the sheet music is supposed to represent.

In many cases, pianists use lead sheets to jog their memory. They know the songs well and know how the recordings sound, and can use that as a guide. But they haven't memorized every single chord and thus use the lead sheet as a memory aid when they forget a chord or two. In other cases, a band might start playing a tune that the pianist has never heard of. The lead sheet provides the essential bones/structure from which the pianist can then improvise his/her part to match the dynamic of the group he/she is playing with.

So a jazz pianist likely isn't improvising everything when he/she looks at a leadsheet. Certain things are remembered from recordings, while other things are created spontaneously. This isn't as intimidating a feat as it sounds--one achieves this on-the-spot improvisation by drawing very heavily on the practice one has done. Taking rhythm as an example: one must improvise rhythms to use when playing chords behind the melody. But many of the candidate rhythms comes from a relatively small handful of rhythmic figures that sound good (e.g., dotted quarter followed by an eighth note). So practicing these known rhythmic figures makes the left hand automatic when supporting the right hand and its melody.

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Why choose to describe music whith chord progressions ?

Keep in mind that a lot of Jazz is based on the premise of putting your own spin on existing songs (standards). The melody and chord progression is typically the only part that stays the same between those interpretations. And even those vary.

So lead sheets convey only the parts you need to know—the kernel of the tune. Sometimes that does include more than just chords and melody like an intro or bass ostinato that is an important part of the tune. The rest is open to interpretation.

Is the lack of informations given in a lead sheet the reason why improvisation is such a big part of playing jazz ?

It works the other way around.

In other words, do jazz pianists have to improvise over every bar of their lead sheets, by deciding on the fly of the way they are going to play

Yes, mostly. The way you comp a piece may not be true improv each time. Maybe you know generally what voicings you might play because you've played the tune before. But the rhythm is much more up in the air especially given that you should be interacting with the rest of the band and playing off of their rhythms.

But it doesn't have to be that way when you're starting out. Feel free to write out exact arrangements if that helps you.

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Is the lack of information given in a lead sheet the reason why improvisation is such a big part of playing jazz ?

On the contrary, there is plenty of information present on the lead sheet, and the chord[progression]s tell the soloists what scales are most appropriate when they improvise. As you might expect, the 1, 3, 5 and 7 (chord tones) are the most attractive tones in a melody.

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" Why choose to describe music whith chord progressions ?

I mean, it doesn't carry that much informations. The chords themselves can be play in a million possible ways (with inversions, arpeggios, rythm, voicing, dynamics, etc.). "

And that, in a nutshell, is what jazz is about. Finding the million different things that can be played over a certain chord sequence. 'Jazz' can also include sticking to the composed melody and re-harmonising. Or in going off at tangents only vaguely suggested by the original song. But mostly it's based on the chord sequence. Great jazz musicians invent alternative melodies. Lesser ones just run up and down 'the scale of the chords'.

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