Two notes being played simultaneously is referred to as an Interval. An interval can be considered consonant (strong sounding) or dissonant (strange sounding!).
Broadly speaking, there are two types of intervals: simple intervals and compound intervals. A simple interval is an interval of less than an octave, while a compound interval is an interval greater than one octave.
Lets look again at our major scale, in the key of A:
The root (A) played with the octave (A) is the strongest sounding interval, since it is just two notes of exactly the same pitch, an octave apart. Here are some examples of octaves on the guitar fingerboard:
The second strongest sounding interval is the 1st (A) played with the 5th (E). This is a strong, resolving sound and as such the 5th interval is referred to as a Perfect 5th (or a diminished 6th) This is the interval that power chords are derived from.
The next best sounding interval is the 1st (A) and 4th (D) played together. The 4th interval is referred to as a Perfect 4th (or an augmented 3rd).
THIRDS AND SIXTHS
The next best sounding intervals are the root (A) with either the 3rd (C#) or the 6th (F#). These intervals are not as consonant as the previous intervals. They are referred to as a Major 3rd (or diminished 4th) interval and a Major 6th (or diminished 7th) interval. Note also that each interval of a 3rd can be ‘inverted’ to an interval of a 6th.
SECONDS AND SEVENTHS
More dissonant intervals are created when the root (A) is played with either the 2nd (B) or the 7th (G#). They sound like they don’t really belong together, but are useful in music since they add colour to a chord and suggest motion when used in a progression. They are referred to as MAJOR 2nd (or diminished 3rd) and MAJOR 7th intervals.