The key signature of a musical composition defines the tonal center and basic scale formula and is identified by a series of sharps and flats placed directly after the clef on a music staff. The most prominent key signatures are major and minor keys, but there exist also modes. The use of key signatures is to aesthetically simplify the score to a piece of music by providing the notes that will most often be sharps or flats without needing to write an accidental for each and every instance of that note in the piece or following section.
The sharps and flats in a key signature must be notated in a very specific order. The first sharp to appear is F-sharp, the following sharps are each a perfect fifth above the previous pitch. The first flat will always be B-flat, with the following pitches each being a perfect fourth above the last.
How to identify the key of a piece
Use the table provided to identify which key signature indicates which pitches. For instance, B-flat major is always notated using two flats—B-flat and E-flat consecutively. The tonal center is always a perfect fourth below the very last flat used in the signature for flat keys (a perfect fourth below E-flat is B-flat). However, the same key signature is also used for minor keys. The relative minor key is always a minor third below the tonal center for the major key. For sharp keys it is a very similar identification process. Take A major for instance, which comprises of three sharps, F-sharp, C-sharp, and G-sharp in that order. The tonal center for sharp keys is a minor second above the last sharp in the key signature (a minor second above G-sharp is A). The relative minor is still a minor third below the tonal center for the major key—in this case a minor third below A is F-sharp, so three sharps can mean either A major or F-sharp minor just like two flats can mean either B-flat major or G minor.
What key should I write my piece in?
This depends on several factors, including but not limited to
- what is idiomatic for the instrument you are writing for,
- the skill level of the performers you expect to play your music,
- what type of colour you would like to achieve,
- what kind of mood you would like your piece to be in.
In order to write idiomatically for the instrument, you must understand what the instrument you are writing for is capable of doing in the hands of a performer at the level at which you expect your piece to be performed. If you are writing for a professional musician or group, you have more freedom in which keys they can play in, while if you are writing for a typical high school band you would be smart to keep the piece in "easy" keys for band instruments. To find out more about which keys are appropriate for what ensembles, see Category:Ensembles. To investigate which keys specific instruments are most suitable for, see Category:Instruments.
If you are writing for a piano or organ, it is very unlikely that nuances in tone colour will be noticed. Therefore F major and G-flat major will almost sound identical aside from the actual pitch difference. Thus when writing for piano, organ, or instruments in a similar quality the composer relies on other reasons to choose one key over the other. For amateurs, F major is easier to read. For professionals, G-flat major feels better because it is slightly more ergonomic.
For wind and stringed instruments, tone colour is more apparent and inherently more important in deciding which key is appropriate for your piece. This involves having an exhaustive knowledge on the specifics of each and every instrument you are writing for. For instance, the alto saxophone is very suitable to playing in flat keys such as concert F, B-flat, E-flat, and A-flat major (and their relative minors) because the instrument itself is pitched in the key of E-flat and the above mentioned keys have the fewest sharps or flats in the key signature. Adversely, writing for alto saxophone in the keys of B major, E major, or D major yields a different tone quality because the pitches for these scales aren't in line with the quality of the instrument as a whole and more sophisticated fingerings may sometimes be necessary. For amateurs sophisticated fingering can be very limiting and high school and community ensembles may tend to play out of tune when asked to play in a key outside of their comfort zone. For stringed instruments, the open strings stick out and writing in keys such as D major and G major will allow the players to take advantage of this. Many pieces in G major and D major as a result are very bright sounding, but keys that avoid open strings altogether, such as G-flat major and B-flat minor will be very dark and warm.
Historically, many keys were identified with specific moods that varied surprisingly little from person to person. E-flat major was always considered an "epic", "grand", "majestic", or "heroic" key for orchestras, while D major was ruled as "glorious", "bright", and "royal", characterized by fanfarical trumpets and other brass. For information on the moods often associated with specific keys, see their individual articles at Category:Keys.