What is a third?

Answer 1

That's a vague sounding question, but I've encountered the term in Music Theory, referring to the third step of an 8 step scale (do, re, mi). The 3rd is known as a color tone along with the 6th, 7th, and 9th, because it helps to determine the quality of a chord (more than two notes played in unison).

A triad is a 3 note chord consisting of the 1st, 3rd, and 5th degrees of a scale played in unison. It can be major, minor, or diminished depending on the relative distance between scale degrees.

Every scale in western music contains 12 half steps. This constitutes an octave, or the repetition of the root note one octave up. The 3rd can determine whether a chord is major, minor, or diminished by its location between the root note and the 5th. If a 3rd is three half steps from the root and four half steps from the 5th, a minor chord is made. If it is four half steps from the root and three half steps from the 5th, a major chord is made. If the 3rd is three half steps from the root and three half steps from the 5th, a diminished chord is made.

It is possible to create major and minor intervals with only two notes. Three half steps between notes indicates a minor interval, and four half steps indicates a major interval. A diminished chord requires at least three notes, as it cannot be diminished without a flat 5th.

I hope this quick breakdown of triad music theory sheds some light.