Key Signatures

The key signature defines a piece of music's tonal centre and indicates which notes should be raised or lowered. All music is in a certain key, each scale is in a certain key and each chord is in a certain key also. This key defines what is called that piece of music's 'tonality' or 'tonal centre'. Likewise, it defines each scale and each chord's 'tonality' or 'tonal centre' too. 

The key signature tells us what tonic note all the other notes in a piece of music, scale or chord are related to. For example, a melodic line in a tune that takes its notes from the scale of A major is in the key of A major. The tonic note is the first note of the scale and therefore the key-note which in this example case is A. Likewise, a melodic line in a tune that takes its notes from the scale of G minor is in the key of G Minor. The note of G is the key note and all the other notes in the melody will all relate to the note of G in this particular case.

Now, whether the scale and the relative piece of music is in the key of A major or A minor all depends upon the other notes in the scale and what notes are indeed permitted to be played or not be played in that scale and its relative piece of music. Remember, all music is related to a particular scale that starts on a particular note, the tonic. The tonic note, as we have established, is the first note of the scale and gives the scale its tonal centre and us some indication of what key it is in.

A key signature is not the same as a key. It is merely the notational illustration to give the music reader information about the specific musical piece in question. It tells us the key that the music is in but it is not the key, much in the same way that a person's signature is not the person rather it identifies the person. They are ultimately only really necessary for those who are learning to read or who are already reading music or for those who are arranging or composing music that will be written down on a music staff. When a piece of music is written on the staff the key that the music is written in is clearly defined by showing what notes should be raised by one semi-tone (sharpened) or lowered by one semi-tone (flattened).

The diagram on the right illustrates the key of A major. A major has three sharps - C#, F# and G#. It also has a relative minor key which is F# minor. All major keys have a relative minor key and all minor keys have their major relative too. This will be discussed further down. The illustration seen here is a 'key signature'. It shows the musician/reader what sharps are required to be played for the piece of music to which it relates and therefore what key the music is written in.

Music often changes key, sometimes for the remainder of the piece being played, or just for a short while before reverting back to the original key set out from the beginning. If the music changes key for some length of time then it will be written with a new key signature for the duration of that new key. When the piece being played then reverts back to the original key, the original key signature is written once again to alert the musician of the change of sharps to be played or not be played.

Now, if on the other hand there is a short, only temporary change of key, it will be illustrated with individually written sharps (#) or flats (b) next to the specific notes to be altered at the required point in the written music as shown in the first diagram below. 

The staff to the right illustrates the B major scale. When no key signature is written these additional sharps or flats are called accidentals and would only be played when illustrated as shown here.

By Wahoofive via Wikimedia Commons CC-BY-SA-3.0

The diagram to the right illustrates the same B major scale but with a clearly written key signature at the beginning of the staff letting us know what sharps to play.

By Wahoofive via Wikimedia Commons CC-BY-SA-3.0

The diagram above tells us that the music (which in this case is the scale of B major) has 5 sharps - A#, C#, D#, F# and G#. No other A, C, D, F or G (a flat or a natural variant) should be played unless we are instructed to do otherwise during the course of playing the piece.

In addition to the above is the natural sign as seen in the second measure of the staff below. The natural sign indicates to the musician when to revert back to a natural note or a number of natural notes as opposed to the sharp alternatives that were previously played.

The staff to the right shows two measures. The first is in the key of A#/Bb minor which contains seven sharps no less as clearly indicated by the key signature at the beginning.

By Hyacinth at the English language Wikipedia [GFDL

The second measure (or bar) above changes the key to A minor which contains no sharps or flats and therefore instructs the reader/musician to now play the previously played sharps played in the first measure as natural notes by dropping them down one semi-tone. This is just one example of many possible variations that can happen at any point during play as the key can change at any time during a piece of written music, although usually at the beginning of a measure as indicated above. In this case the new signature has no sharps or flats and is therefore a signature of natural notes to cancel out the sharps played in the previous signature's measure.

Key Signatures and the Circle of Fifths

All the signatures can be combined and expressed in the diagram below known as the 'circle of fifths'. This table clearly illustrates the relationship between the twelve keys and the major/minor relatives. Around the outside of the diagram are the twelve major keys showing how they would be written on the music staff. The shaded grey area gives the number of sharps or flats in each key. The inside area contains the relative minor key to each major key. The circle of fifths is a visual aid to clarify and to further understand key signatures and their relationships with one another.

By Just plain Bill (Own work) - CC-BY-SA-3.0

The term 'fifth' comes from the fact that each note or key in the circle is separated from the one before and also the one after by a 'fifth'. A 'fifth' is more correctly called a 'perfect fifth' and is found seven semi-tones above or below any note or key. See this for yourself. Play the note of C at the third fret on the open A (fifth string). Now play each consecutive note on the same string moving up one fret (one semi-tone) at a time for seven notes. The seventh note will be the note of G as illustrated in the circle of fifths above.

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