Determining the KeyI am going to show you how to find the key when you have just a written piece of music in standard notation and also when you just know the chord progression.
Determining the Key From a Piece of Music in Standard NotationTo find the key when you see a piece of music in standard notation, you must look at the key signature. The key signature is located at the beginning of the line (for more info on the key signature refer to Reading Standard Notation). This is where sharps and flats in standard notation are placed. Their are either sharps, flats, or neither in the key signature. Sharps and flats are never mixed together. You can determine the key by counting the number of sharps or flats in the key signature as long as the key signature does not have 5 sharps or flats. You must use a more complicated process to determine the key if this occurs. Here is a chart to show you the key if you have 4 or less flats or sharps in the key signature.
Now, to find out which notes are sharp or flat, we will use the circle of fifths.
If you can remember that the 1st sharp is F# and the 1st flat is Bb, then you will have no problem finding the sharps or flats in a key signature. Figure 3 will show you how to find the sharps or flats in a key signature.
Now comes the hard part... finding the key when there are 5 sharps or flats. When there are 5 sharps or flats, it gets tricky because there are 3 keys that use 5 sharps or flats. These keys are B, F#/Gb, and C#/Db. All 3 of these keys have the same 5 sharps or flats. The difference between these keys is the notes that are not sharp or flat. In the key of B, those notes are B and E. In the key of F#/Gb, those notes are B and F. In the key of C#/Db, those notes are F and C. You must know these notes so that you can determine the exact key because the only way to determine the key now is to find those 2 notes used in the song. For example, if you see a B being used, you have eliminated C#/Db from being the key. Then you must seek find either an E or an F to differentiate between the key of B and the key of F#/Gb.
Finding the Key from a Chord Progression
To find the key of a chord progression, you must have some knowledge of chord progressions.
If you recall, certain degrees are major and certain degrees are minor. To find the key from a chord progression, do the following: