Here at UG there are some great lessons on intervals and music theory in general, but I'm going to try and make my interpretation a bit easier for people new to theory. Intervals are a fairly simple concept long as you learn them and practice them. They are only as hard as you make them to be.
Well now that I've made you read a introduction, I'll cut to the chase.
Lets get started!
The fist thing about intervals is knowing your basic musical alphabet. It's what you need to apply your intervals too.
The music alphabet consist of the letters:
A A# B C C# D D# E F F# G G# (This is also called the chromatic scale).
*Notice they're all half steps!
Thats it! Once you hit G# just go back to A and repeat them again.
If your at C, just go to C# and then D and so on..it's the same for anywhere you start. One thing to know/notice is all the intervals have a sharp between them except for B to C and E to F. Those are the two natural half steps (thats what I call them) within the musical alphabet. If you go from A to B then that would be a whole step. If your wondering what A to C or A to D# is (for example), don't worry, thats coming soon.
*There are ways to alter B to C and E to F to make them whole steps but thats a bit more advanced for beginners.
A to B= whole
B to C= half
C to D= whole
D to E= whole
E to F= half
F to G= whole
G to A= whole
Now to get to the intervals.
The first thing to learn is your half step and whole step. These two intervals in turn can be used as your building blocks. I'll get to that a little later. As for applying these two intervals to the guitar, that is simple. On the guitar, each fret represents a half step.
Half Step = 1 half step or fret
Whole Step = 2 half steps or frets
*The half step is also called a m2 or is 1 semi-tone.
*The whole step is commonly called a M2 or is two semi-tones.
The next two intervals, which are very important, are the major and minor 3rd. These two intervals are great for when you begin chord construction.
m3 = 3 half steps or frets
M3 = 4 half steps or frets
When you get familiar with these two intervals you can apply them to make basic triads.
Here are the general "interval formulas" for two of the four common triads. I've also given you the patterns for the Root position on the first 3 strings (G-B-e). If you want some other useful patterns for these two triads and more, then check out my Chord Patterns lesson.
Major Triad = Major 3rd + minor 3rd
Root Position Shape
Minor Triad = minor 3rd + Major 3rd
Root Position Shape
This is one of the many popular uses of 3rds in general. The most popular use for 3rds is for harmonies. Listen to Iron Maiden or the more modern Avenged Sevenfold. They both incorporate harmonies through 3rds and other intervals.
Heres a chart to show you the type of interval and the number of half steps each interval has. I've also included some extra info about each interval.
*Remember when I mentioned A to C and A to D#. Well those are two examples for intervals. A to C is a m3 and A to D# is a tritone. If your wondering how I knew that then use this chart to help you.
If you need a guide use the chromatic scale.
Interval Number of Half Steps
Perfect Unison 0
m2 (half step) 1
M2 (whole step) 2
Perfect 4 5
Tritone (#4/b5) 6 *The tritone is also known as the "Devils" note
Perfect 5 7
Perfect 8 (Octave) 12
Well hopefully this little lesson helped you out for the basic content of intervals. If you'd like more for the other intervals (perfect 5 tritone, etc.) I'd be happy to help anyone.
This is one of my first lessons so don't be too harsh.