Modal theory helps guitarists understand how scales and notes are connected with each other. When a player understands the relationships between notes, they are better able to create effective solos. Modes are basically versions of a scale, which is a series of consecutive notes with a distinct pattern of half-steps and whole steps. Once you understand some basic music theory and how to create scales, you will be able to recognize modes.
Modes have been used in music since the time of plainchant, most commonly referred to as Gregorian chant. Modes were used in the sacred choral music of the medieval times, and they remained popular until the major and minor scales became dominant. Most popular music today follows either a major or minor scale, but many guitarists like to use modes to vary their solos. Modes are popular in shredding and metal style playing.
Scales are made up of a series of notes. They usually start with one note and end with the same note an octave higher. Major scales follow the pattern "whole-whole-half-whole-whole-whole-half." Natural minor scales follow the "whole-half-whole-whole-half-whole-whole" pattern. Major scales are the same as the modal term "Ionian," and minor scales are also called "Aeolian." A "Dorian" scale is the same as a major scale starting on the second scale degree, "Phrygian" is a major scale starting on the third scale degree, "Lydian" starts on the fourth scale degree, "Mixolydian" begins on the fifth scale degree, and "Locrian" stars on the seventh scale degree. You can have modes beginning on any scale degree of any kind of scale; not all the modes have names like the ones given to modes of a major scale. When you use different modes, you are changing the pattern of whole steps and half steps.
The different modes have a quality of sound that may or may not fit the style in which you are playing. Ionian, being simply a major scale, is used most often in most styles of music. Lydian is bright and works well in pop music. Phrygian is dark and popular in heavy metal music. Aeolian, or the natural minor scale, is gritty and works for blues and rock. Mixolydian and Dorian are neither too bright nor dark and are both suitable for rock, pop and country. Locrian is a dark-sounding scale also used in heavy metal. Scales and modes on the guitar tend to follow a particular fingering or fret pattern.
The purpose of studying modes and memorizing their formulas is to be able to use them in improvised solos or to better pick up a solo by ear. If you choose a scale to work with, you know exactly what notes you can use. You may want to study chord structure as well, so when you hear the chords you are soloing over, you know what notes are ideal resting points for your solos. You want to rest on notes in your solo that are present in the chords being played with it.
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