Music Theory: Understanding Key Signatures and Tonality

In our last lesson, we explored the Circle of Fifths and learned that there are twelve major and twelve minor keys, each defined by a unique key signature. For many students, memorizing these can be challenging. Is there a shortcut to help remember all the major and minor keys? And how are these key signatures connected?

Today, we’ll dive into how music theory reveals patterns in major keys.

Key signatures chart | SheetMusicGo

Sharp keys signature

We begin with sharp keys. These are key signatures that contain sharps (#), increasing in number from one up to seven, as seen in the visual chart.

To interpret sharp key signatures, read from left to right, just like reading text. For instance, in this example:

E Major | SheetMusicGo

The order of sharps is F#, C#, G#, D#.

The first rule for sharp key signatures is: Each added sharp is a perfect fifth above the previous one.

Let’s unpack this using examples. Take the first sharp (F#) and the second (C#).

F# and C# | SheetMusicGo

Although they appear reversed on the staff, by adjusting F# down an octave, the interval between them becomes a perfect fifth—something we previously covered in music theory basics.

F# and C# | SheetMusicGo

The pattern continues: From C# to G#, the distance is again a perfect fifth. (C–D–E–F–G).

C# and G# | SheetMusicGo

This rule remains consistent even in the piano scale that contains all seven sharps. When notes appear lower, just shift the octave to make the fifths clearer.

C# Major | SheetMusicGo

Once you know the key signature, how do you determine the key name?

The second rule: In sharp keys, raise the last sharp by a half step to get the major key.

Let’s try it. This key signature has two sharps: F# and C#. The last is C#, and raising it a half step gives us D. That means the key is D major.

D Major | SheetMusicGo

One more: With five sharps—F#, C#, G#, D#, and A#—the last is A#. Go up a half step and you get B, so the key is B major.

B Major | SheetMusicGo

Now, let’s switch gears to flat keys.

Flat signature Chart | SheetMusicGo

Flat key signatures

Flat key signatures use the symbol (♭) and also range from one to seven flats. As with sharps, flats follow their own pair of rules.

The first rule in flat keys is the reverse of sharp keys: Each new flat is a perfect fifth below the one before it.

Flat Keys | SheetMusicGo

Example: Compare the first flat (Bb) with the second (Eb). This interval is a perfect fifth going downward, following the same music theory principle.

Bb and Eb | SheetMusicGo

And yes, even the piano scale with all seven flats follows this rule. Again, adjust the octave if needed to maintain consistent fifths.

7 flat key | SheetMusicGo

The second rule for flat key signatures: The second-to-last flat gives you the major key name—except for F major, which is the one exception.

Try this one: A key signature with Bb, Eb, and Ab. The second-to-last is Eb, which means the key is Eb major.

E Major | SheetMusicGo

One more: With six flats (Bb, Eb, Ab, Db, Gb, Cb), the second-to-last is Gb, making the key Gb major.

Gb Major | SheetMusicGo

Now, how do minor keys fit in?

Minor keys are directly linked to major keys in the Circle of Fifths. Both share the same key signature and are called relative keys. The circle shows these pairs clearly, usually with major keys on the outside ring and minor keys on the inside.

Circle of Fifths | SheetMusicGo

You can find the relative minor key by lowering the major key by a minor third.

g minor | SheetMusicGo

For example: If the key signature has two flats (Bb and Eb), the major key is Bb major. Drop it down a minor third, and you land on G. So, the corresponding minor key is G minor.

The relationship between major and minor keys, along with how sharps and flats build across the piano scale, all tie back to the elegant logic of music theory.