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While reading and refreshing basic music theory, I've met the tonic note as related to common practice diatonic scales.

From introductory resources, the usual explanation is simple enough. The first (the lowest) note of the scale is the tonic note; the most important note of the scale, the centre around which a melody tends to be oriented, and where it feels resolved.

However, I've never seen any further justification of why the first note of the scale is the tonic. It's just usually stated for the reader to accept.

Given my limited knowledge and reading, I can think of two possible explanations (that may well be far off the mark):

  1. The choice is somewhat arbitrary. When we construct a scale on a note, there's a convention that the first note will be most important in the composition.

    A composer then goes about deliberately following this rule and establishing that first note as most important. And, maybe, through continued exposure to music constructed this way, listeners expect certain scales to tend to particular notes.

  2. There is actually a fundamental underlying physical or perceptual reason why the first note is the most important. It's not just an arbitrary choice.

    I can certainly hear that the tonic behaves this way, e.g. when a scale is played, it feels resolved when you reach the starting note one octave up. Perhaps this is due to particular interval relationships, or some other effect? (Or maybe this is an effect of familiarity, as mentioned above in 1...)

  • Chicken and the Egg: Tonic does not mean "most important". I assert that it's importance is earned and imparted into the words meaning after centuries of use. – amalgamate Nov 10 '16 at 23:39
  • Just start your scale playing from a different note in the scale. You will notice a difference in color. For example, in the G major scale, if you start playing from the C note, it will sound different that just playing the C major scale... but will sound kind of like C major with a different color. A thing called modes. – blusician Nov 18 '16 at 7:33
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The first note of a scale is the tonic (or root of the key) by definition. We build scales off the tonic; we do not define the tonic based on the scale.

The tonic itself comes from the key the piece is based in. The key is defined somewhat by the accidentals, but also by the harmony of the piece. You'll run into music theory words like cadence here. You hear this theory as the resolution that you mention in your answer.

Regarding the eighth note sounding resolved, it might be worth mentioning that octaves have a specific frequency relationship. The frequency is exactly double the note an octave below. Our ears hear this as the same pitch class. So there are some physical reasons at play.

  • "We build scales off the tonic; we do not define the tonic based on the scale" well said – Unnamed Sentient Being Nov 11 '16 at 1:59
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Endorph's answer is correct. I just want to add a little bit to address the possibilities you mention. Both of your possibilities are, the most part correct.

Recent studies in music perception seem to confirm the idea that music is arbitrary in many ways. With reference to your question regarding the tonic, you can think of it as arbitrary in that any note can be a tonic and you can use any scale. It is up to the composer. Tonic and scale are independent of each other. You propose the idea that certain scales are linked with certain notes. This is not necessarily true. Any scale can be played from any note and likewise any note can be the tonic of any scale. There are sometimes other factors which determine the choice of tonic or scale. For example, all singers and instruments have a limited range. If a piece of music is outside of that range you came transpose it to a range that would be more suited. From one perspective this is basically changing the tonic of the piece. Another example is if you have a song using a major scale, which by our western tradition is associated with a happy sound, and you want to change the mood of the piece to a sad sound, you can switch the key to a minor key. This is essentially changing the scale the piece uses but keeps the same tonic. All arbitrary.

Your other possibility, that there are physical reasons for these choices, is also correct in a way. As I mentioned there are sometimes physical limitations of the instruments. But also the traditions of western classical music are grounded in the contrasts between dissonance and consonance, which can be analyzed in terms of sound waves and acoustics and harmonics. So once the tonic and scale have been arbitrarily decided the composer uses the qualities of dissonance and consonance (and many other techniques) to move the listener away from the tonic and then ultimately back again, in ways that the culture has come to understand.

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