Music theory · 6 min read

Tuning systems and temperament, explained

Pure intervals do not fit into an octave. How just intonation, meantone and equal temperament each solve that, and what it means for reeds, strings and ears.

2026-08-24

Where intervals come from

A vibrating string or column of air produces not one frequency but a series: the fundamental, then twice that frequency, three times, four times, and so on. These are the harmonics, and the simple ratios between them are the origin of consonance. Double the frequency and you get an octave, ratio two to one. Three to two gives a perfect fifth. Five to four gives a major third.

Intervals built on small whole-number ratios sound smooth because the harmonics of the two notes coincide. When the ratio is slightly off, nearby harmonics interfere and you hear beats, a slow pulsing in the sound. Beats are not an abstraction; they are the practical tool every tuner uses, counting the pulses per second to set an interval.

So far so tidy. The trouble begins when you try to build a whole keyboard from pure intervals.

The problem that forces a compromise

Stack twelve pure fifths, each at a ratio of three to two, and you should arrive back where you started, seven octaves higher. You do not. Twelve pure fifths overshoot seven octaves by a small but audible amount, about a quarter of a semitone, known as the Pythagorean comma.

There is a second and independent mismatch. Four pure fifths should give you a major third two octaves up, but the third you arrive at is noticeably wider than the pure five-to-four third. The gap is called the syntonic comma, and it is the reason a keyboard cannot have both pure fifths and pure thirds.

These are arithmetic facts, not historical accidents. No tuning system can make every interval pure on a fixed-pitch instrument. Every system is therefore a choice about which intervals to protect and which to sacrifice.

The main solutions

Just intonation keeps intervals pure within a single key. The thirds are beautiful and the fifths are clean, and the moment you modulate somewhere distant the errors accumulate and one interval becomes badly out of tune, the so-called wolf. It is excellent for unaccompanied voices or for music that stays home, and impossible as a general keyboard solution.

Pythagorean tuning builds from pure fifths and accepts wide thirds. It suits medieval music, where thirds were treated as unstable anyway, and sounds harsh in later repertoire.

Meantone temperament, dominant from roughly 1500 to 1700, narrows each fifth slightly to make the thirds nearly pure. Common keys sound very sweet, remote keys become unusable, and the surviving wolf fifth is genuinely unpleasant. This is why early keyboard music lives in a small set of keys.

Well temperament, associated with the early eighteenth century, distributes the error unevenly so that every key is playable while each retains a slightly different character. Bach's Well-Tempered Clavier belongs to this world, and the point of writing in all twenty-four keys was partly that they did not sound identical.

Equal temperament divides the octave into twelve exactly equal semitones. Every fifth is narrowed by about two cents, which nobody notices, and every major third is widened by about fourteen cents, which is audible as a slight restlessness if you listen for it. In exchange, every key is equally usable and modulation is unrestricted. It became standard during the nineteenth century and is what almost all modern instruments use.

Accordions are tuned, and then tuned again

An accordion reed is a steel tongue riveted over a slot, and its pitch is set by filing metal from it: from the tip to raise the pitch, from the base to lower it. Once set, it stays set. There is no adjustment in performance, which is why accordions are built in equal temperament and stay there.

What makes accordion tuning unusual is that instruments often carry two or three reeds sounding the same written note, and they are deliberately not tuned identically. Offsetting one reed by a few cents produces beats between them, and that beating is heard as warmth, shimmer or, at wider offsets, the characteristic musette sound of French and Italian playing. The amount of offset, called tremolo or wet tuning, is a stylistic decision: dry for classical and Russian bayan playing, progressively wetter for folk and musette.

Temperature moves reed pitch, and a hot room will raise an accordion slightly. This matters when playing with fixed-pitch instruments and is one reason a cold instrument brought into a warm hall needs time to settle before anyone checks the tuning.

The double bass adjusts constantly

A string player has no fixed pitches beyond the open strings, so intonation is a live decision on every note. Bass strings are normally tuned in fourths: E, A, D, G from the bottom. Tuning those fourths by ear means listening to the harmonics on adjacent strings until the beats stop, which sets them pure rather than equally tempered, a small discrepancy that players simply absorb.

In practice a bass player adjusts to context. Playing a major third within a sustained chord, you will find yourself lowering it slightly towards the pure interval because the beats are audible in a way they are not on a piano. Playing a leading note in a melodic line, you will tend to raise it towards the tonic. These two instincts pull in opposite directions and the good player chooses according to what the passage is doing.

Playing alongside a fixed-pitch instrument removes that freedom: you match the keyboard, because the alternative is beating that everyone can hear. That is a routine adjustment for any bass student at A&G Academy who works with accordion or piano, and it is worth saying aloud in a lesson rather than leaving them to discover it in a concert.

Frequently asked

Why can't all intervals be perfectly in tune?

Because the pure ratios do not divide the octave evenly. Twelve pure fifths overshoot seven octaves by roughly a quarter of a semitone, and pure thirds conflict with pure fifths as well. Any fixed-pitch instrument must therefore mistune something. Temperaments differ only in which intervals they protect and which they compromise.

What is equal temperament?

A tuning that divides the octave into twelve mathematically equal semitones. Every fifth is very slightly narrow and every major third noticeably wide, but the error is identical in every key, so all keys are equally usable and any modulation is possible. It became standard in the nineteenth century and is the modern default.

Why do some accordions sound wavy?

Because two or more reeds sound the same note with one tuned a few cents off the other. The small frequency difference produces beats, heard as shimmer or waver. A wide offset gives the musette sound of French and Italian folk playing; classical and bayan instruments are tuned dry, with little or no offset.


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