As you study guitar theory, you'll hear terms like mode, tonic, and scale. The most common scale in music is the major scale. It includes seven degrees, or pitches, and involves seven steps or intervals. When you play the major scale beginning on its 1st degree, you create the familiar "Do, Re, Mi . . ." sound. But the starting position of the major scale isn't always the 1st degree.
What is a mode?
Music often centers on other degrees in the major scale besides the 1st one. For example, start a major scale on the 6th degree and you create what is known as the minor scale (also known as the relative minor or natural minor). The 1st and 6th degrees of the major scale produce the major and minor scale. In music, you say that the scale has these two different modes.
The major scale has more modes than just the 1st and 6th degrees. The truth is, any scale degree can be used as the starting point. However, the major and minor modes are the most commonly used scales in music. In fact, they're so common that they're not usually thought of as modes. Instead, they're thought of as plain or natural scales. It's only when music centers on one of the other degrees in the major scale that the music is considered modal.
Of modes and tonics in the major scale
There are seven degrees in the major scale, and each one can function as the tonal center, or first tone (tonic) of the scale. Long ago, the Greeks named each one of these modes. The names are as follows:
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I. Ionian: More commonly known as the plain major scale.
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ii. Dorian: A type of minor scale with a major 6th. Fairly common. "Oye Como Va" by Santana is an example of a song that centers on the 2nd degree of the G major scale using the chord progression Am7-D9. You say that it's in A Dorian mode.
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iii. Phrygian: A type of minor scale with a flattened 2nd. It has a Spanish flavor to it, but it's not used much. One example is "The Sails Of Charon" by Scorpions, which centers on the 3rd degree of the G major scale, producing B Phrygian mode.
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IV. Lydian: A type of major scale with a sharpened 4th. Occasionally used temporarily in a song before the music settles on a more stable tonic, such as I. Listen to "Man on the Moon" by R.E.M for an example of C Lydian, the 4th mode in the G major scale.
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V. Mixolydian: A type of major scale with a flattened 7th. Very common, almost as much so as the plain major scale. You should recognize this mode anytime you start on a major chord and then move down a whole step to another major chord. A change like that is almost always V-IV. "Seven Bridges Road" by the Eagles centers on the 5th degree of the G major scale, producing D Mixolydian mode.
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vi. Aeolian: More commonly known as the natural or relative minor scale.
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vii. Locrian: Not used. The 7th triad has a flattened (or diminished) 5th making the chord too unstable to function as a usable tonic.