Infinite Lounge
What is a jazz piano trio? Who plays what, and who listens
Piano, double bass, drums. What each of the three actually does in a swing tune, how the swing feel changes with tempo, and why a trio that doesn't listen to itself sounds like three tracks rather than a band.
Three players, one job each, and then some
A jazz piano trio is piano, double bass and drums. The roles are clear-cut on paper: the piano carries melody and harmony, the bass carries the root and the pulse, the drums carry the time. What makes it a band is that none of them does this in isolation.
Piano: two hands, two jobs
The right hand plays the melody and then improvises lines over the chords. Bebop lines put chord tones on the beat and use scales with an added passing note to make that work: a dominant chord gets the Mixolydian scale plus the major seventh, a minor seventh chord gets Dorian plus the major third.
The left hand plays the chords, and usually not with the root. Because the bass already has the root, the left hand plays a rootless voicing: just the 3rd, 5th, 7th and 9th (on a dominant chord, the 13th replaces the 5th, which carries the least information). These sit roughly between C3 and A4, under the right hand and above the bass.
And the left hand does not hit a chord on beat one of every bar. Real comping is syncopated, and it often anticipates: the next bar’s chord arrives on the last off-beat of the bar before. When the right hand gets busy, the left hand plays less, and it tends to fall in the right hand’s rests.
Bass: the root and the walk
In a swing tune the bass walks: four quarter notes a bar, the root on beat one, chord tones on beats two and three, and on beat four a note that leads into the next root, usually a half step above or below it. That approach note is why a walking line seems to know where it is going.
The bass does not always walk. During a tune’s head it often plays in two, on beats one and three only, and saves the walk for the solos. In a waltz it deliberately does not fill all three beats; fill them and you get an oom-pah-pah Viennese waltz. It also leans on beats two and four slightly, to lock with the drummer’s accents.
Drums: the ride cymbal is the clock
The swing drummer’s time lives on the ride cymbal: quarter notes as the skeleton, with “skip” notes on the swung off-beats of two and four as decoration. The accents fall on two and four, not one and three, and the hi-hat pedal closes on two and four too. Snare and bass drum are not a pattern; they are reactions.
On ballads and slow blues the sticks give way to brushes, and brushes are a different instrument: the core of brush playing is a continuous bed of friction noise swept across the snare, swelling toward two and four, not a series of hits.
The swing feel changes with tempo
“Swing eighths” are not a fixed triplet. Research cited by the engine (Friberg & Sundström, 2002) found the short note of each long-short pair stays at roughly 100 milliseconds whatever the tempo, so the ratio is wide when the tune is slow and nearly even when it is fast. Soloists swing lighter than the ride.
| Tempo | Ride and bass | Piano line |
|---|---|---|
| 120 BPM and below | ≈ 3.5 : 1 | ≈ 1.7 : 1 |
| 168 BPM | ≈ 2.6 : 1 | ≈ 1.5 : 1 |
| 240 BPM | ≈ 1.5 : 1 | ≈ 1.2 : 1 |
There is also a timing offset. The piano sits slightly behind the beat on downbeats, while its off-beats line up with the rhythm section.
Six feels
| Feel | Tempo (BPM) | Bass | Drums |
|---|---|---|---|
| Medium swing | 120–168 | walking bass | ride cymbal |
| Up-tempo swing | 196–264 | walking bass | ride cymbal |
| Ballad | 58–76 | 1–2 notes a bar | brushes |
| Slow blues | 62–84 | two-feel | brushes |
| Jazz waltz | 140–200 | waltz bass | waltz ride |
| Bossa nova | 124–148 | root and fifth, syncopated | straight eighths + clave |
Listening is the actual job
If the three parts are generated independently, you hear three tracks, never a band. A real rhythm section does not fill in by a density setting; it responds. The drummer hits with the piano’s accents, nods at the end of a phrase, and fills a gap the piano leaves, and only one of the two answers any given gap. A bar where the piano suddenly gets much busier is something the others hear and react to.
How a tune is laid out
- Intro: often the last four bars of the form, to set up the top.
- Head: the theme, bass in two.
- Solos: piano choruses over the form, bass walking, intensity rising chorus by chorus in terraces rather than a straight ramp.
- Trading fours: piano and drums alternate four-bar phrases late in the tune.
- Ending: perhaps a tag (the last phrase played three times), a slowing over the last bars, a held chord with the bass on the root, a cymbal. Not a fade-out.
Questions
What instruments are in a jazz piano trio?
Piano, double bass and drums.
What is a rootless voicing?
A left-hand piano chord that leaves out the root (the bass plays it) and uses the 3rd, 5th, 7th and 9th, with the 13th instead of the 5th on dominant chords.
Why are accents on 2 and 4 in swing?
The ride cymbal and hi-hat accent beats two and four, and the bass leans on them slightly to lock with the drums.
What is a walking bass line?
Four quarter notes a bar: the root on beat one, chord tones on two and three, and an approach note on four leading into the next chord's root.
- Every number on this page is a value in the Infinite Lounge engine, a rule-based jazz piano trio that improvises in real time: chord voicings (Theory.swift), comping and drums (CompDrums.swift), bass (Players.swift), swing and tempo (Groove.swift), tune structure (Render.swift).
- Swing-ratio rule (the short note of each pair stays near 100 ms): Friberg & Sundström, 2002, as cited in the engine. Soloists swinging lighter than the ride (about 1.3–1.7 : 1): Benadon, 2006, as cited in the engine. The ratios in the table are computed from the engine’s formula.
Also in this series
Infinite Lounge is a piano trio that improvises live on your phone from these rules: no recordings, no network, no training data. Set after set, improvised as it plays.