A 'Scale' can be defined as a series of notes, arrange in a specific order of distance in between them. Each note is separated by a combination of a 'Tone' and a 'Semitone' but take note, that a series of notes separated only by a tone or a semitone apart, will also be considered as a scale.
On the guitar, a 'Tone' is often referred to as a 'Whole Step', achieve by moving two frets away from the original note, while a 'semitone' is referred to as a 'half step', achieve by moving one fret away to the next fret.
There are basically seven notes that form a scale in music, and each note is identify by the letters of the alphabet which is; A, B, C, D, E, F and G. It's important to understand that there will be spaces in between some of these notes (check the fretboard diagram to confirm this). These spaces will be fill by sharps(#) and flats(b), thus creating a series of notes which are called, the 'Chromatic Scale'.
The major scale chooses it's notes from this scale by following a fix order of distance that can also be thought of as a 'Pattern', and the pattern of which a major scale is based upon are as follows:
Tone - Tone - Semitone - Tone - Tone - Tone - Semitone.
Here's a clearer example starting from the note 'C':
Try playing these notes on your guitar, and you should hear a very familiar sound of Do - Re -Me -Fa - So - La -Ti -Do. The above example is a 'C Major Scale', taking its name from the note that it started from. The starting note of a major scale is also known as the 'Tonic' note.
You can create a different major scale by picking a different tonic note to start with, provided that you keep the exact same pattern of tones and semitones. Below are some more major scales:
In these other major scales, sharps(#) and flats(b) are needed in order to stay with the exact major scale pattern (otherwise it wouldn't sounds like Do - Re - Me...)
What's the benefit of knowing this? Well, the most common chord progression is based around the 'Scale Tone Chords' of a major scale, and knowing that is a great asset to help you figure out the chords sequence of your favorite songs. More on this will be discuss in 'Scale Tone Chords'.
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(A - A#/Bb - B - C - C#/Db - D - D#/Eb - E - F - F#/Gb - G - G#/Ab - A)
*'Chromatic Scale' starting from the note 'A'*
T T S T T T S
/ \ / \ / \ / \ / \ / \ / \
C D E F G A B C
T T S T T T S
/ \ / \ / \ / \ / \ / \ / \
G Major = G A B C D E F# G
D Major = D E F# G A B C# D
A Major = A B C# D E F# G# A
E Major = E F# G# A B C# D# E
B Major = B C# D# E F# G# A# B
F Major = F G A Bb C D E F
Bb Major= Bb C D Eb F G A Bb