Music theory is a vast subject. There are so many things to learn and it can be hard to know where to begin. Let’s start by learning what a “key” is. We’ve all heard the term “playing in key” or this song is in the ” key of C” etc…
What does this actually mean? A key refers to the notes in a major scale. So if a song is in the “key of C”, it means that the notes and chords are from a C major scale. Major scales consist of 7 different notes. All keys (major scales) are proportionally the same, meaning there is a relative distance between each note that stays the same no matter what key you are playing in.
Below are the notes in some of the most common major keys (major scales). Notice that each key has 7 different notes. Every key (major scale) has the same exact formula. Between the 1st an 2nd notes in every key there is a whole step then a whole , 1/2 , whole , whole , whole , 1/2 step. So no matter what key you are playing in the distance between the notes is the same. So the formula for a major scale is- W W 1/2 W W W 1/2.
(On a guitar or any fretted instrument a whole step is 2 frets, a 1/2 step is one fret).
Let’s try an example –
The 1st 7 notes of “Mary Had a Little Lamb” in the key of C are – EDCDEEE. In scale degree numbers this would be – 3212333. Now apply this formula to the key of G and you get- BAGABBB. Try it out in all of the keys (major scales below). It’s all the same thing.This is called transposing.
Now you know about keys and a major scales.
I hope this helps.