Scales on any instrument are a way of organizing and remembering the intervals, or spaces, between notes for a certain type of sound. A scale is divided into tones and semitones. On a guitar, this is visually represented on the fretboard. If you play a note and then play another note two frets up from that note on the same string, you have played a tone. If you play a note and then play another note one fret up from that, you have played a semitone. By playing predetermined intervals of tones and semitones from one note until you reach its octave on the guitar, you are playing a scale.
Octaves are the point in a scale where you return to the first note you played, but at a higher pitch or lower pitch depending on if you are moving up or down the scale. On a guitar, the 12th fret is the exact halfway point of the strings' lengths. This is the point on the fretboard that all of the strings reach the next highest octave simultaneously, meaning that after the 12th fret the entire fretboard is starting over. Octaves are also found in various places when two strings are played together, such as the fifth fret on the sixth string and the seventh fret on the fourth string.
The major scale, also known as the Ionian mode, is the primary order of tones and semitones that a guitarist must learn before attempting to learn the remaining six modes. To play the major scale on a single string, place your finger on a note and play the following intervals: tone-tone-semitone-tone-tone-tone-semitone. After playing the final semitone, you will be at the octave of the first note you played. If the first note you played was a C, you would have played C, D, E, F, G, A, B, C.
There are seven different scales, or modes, based on the intervals of the major scale. These are the Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian and Locrian modes. One way to think of these Greek names is to remember that they denote where you should begin playing your intervals from on the major scale. To play a Dorian mode scale, make the second interval of the major scale the first, meaning that the order of intervals would then become tone-semitone-tone-tone-tone-semitone-tone. Notice that the first interval is moved to the end in this example. The Phrygian mode would place the third interval of the major scale as the first one you played and so on for each successive mode.
The two most important modes for a guitarist to learn are the Ionian mode and the Aeolian mode, which is commonly called the minor scale.
Because it is not practical to play scales linearly on a single guitar string, guitarists learn scale patterns that move vertically across all six strings. Many books and guitar teachers break up the notes on the entire fret board for a given scale into five patterns. Each pattern covers four to five frets on the guitar. and should be memorized individually before attempting to put them together. Though the patterns all look different, it is important to remember that the order of the intervals between the root note of the scale and its octave always remain the same.
There are countless guitar books and lessons on the Internet that draw out patterns for the entire guitar neck in any known scale