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Key signature
Musical theory reminder

The key signature enables you to define, within a score, a change of tonality i.e. which scale is used to play the part.
Here is the list of all notes which can be played within an octave :
 

 

1 2 3 4 5 6 7 8 9 10 11 12
C C# 
or Db
D D# 
or Eb
E F F# 
or Gb
G G# 
or Ab
A A# 
or Bb
B
Each of these notes is a semitone lower than the next one.
A major scale has seven notes, with irregular intervals between them : from the root note of the scale, the notes included are located at semitones +2, +4, +5, +7, +9 and +11, which gives, for the C major scale, semitones 1, 3, 5, 6, 8, 10, 12 i.e. the scale C, D, E, F, G, A, B.

For the major scale of D, it will give a scale made of semitones : 3 (root note), 5, 7, 8, 10, 12 and 14.
Semitone # 14 is in fact the 2nd semitone of the next higher octave.
It gives then the scale D, E, F#, G, A, B, C#.

In a score, if only the  notes belonging to the scale of  D are used, then all Fs and all Cs  will be sharp.
To make the notation less cluttered, these two sharp symbols are drawn only at the clef, that is just after the clef symbol. This informs the performer of the key  being used, and in so doing , the default accidentals (notes with sharps or flats).
To determine which key is used in a score (or a part), just count the number of sharps or flats drawn after the clef :

Number of Sharps     Key      Number of flats      Key 
0
C
 
0
C
1
G
 
1
F
2
D
 
2
Bb
3
A
 
3
Eb
4
E
 
4
Ab
5
B
 
5
Db
6
F#
 
6
Gb
7
C#
 
7
Cb
The group of these signs placed after the clef constitutes the key signature.

Generally, key signature changes apply to all staves in the score. Some instruments such as the clarinet, saxophone, trumpet or horn do not play the note actually indicated on the score, however, but a note shifted up or down by a given number of semitones.
They are called "transposing instruments".
 

Tip : 
In the software, to define a staff for a transposing instrument, use "Staff>Apply transposing instrument" or the option "Apply transposing instrument" in the staff contextual menu .
 
For example, on the staff for soparano clarinet, notes are played two semitones below the note actually written .
If the note C is written in the score, the clarinet thus plays a Bb. It is a Bb transposing instrument.
To make a clarinet play an ascending scale of C, i.e. the notes C, D, E, F, G, A, B, you must write D, E, F#, G, A B, C#, i.e. a D major scale.
If the whole score is in the key of C major, i.e. with no key signature, the clarinet staff will have a key signature with two sharps, as if it was in the key of D major.
Note : 
The software extends the notion of transposing instrument to any instrument which does not  play the exact written note.
Thus, instruments like the piccolo or bass, which are not strictly speaking transposing instruments because they simply play the written note shifted by a given number of octaves, are included in the same option .
 
Location

A key change is always positioned at the beginning of a bar.You can make a change of key signature at any bar in the score.
For example, a staff can begin in the key of C major, and then switch to F major a few bars later.
 

Editing

Global editing:
To change the global key signature of a score, select  "Score>key and time signature". The key signature selection box opens.

Local editing :
A dedicated tool palette ("Window>Clef & signature tools") is available. It contains the key signature change tool, as well as tools for changing clef and time signature.

Select the  key signature change tool (icon with sharps symbols on a staff) and click on a bar. The key signature selection box opens.

Selecting a key signature

The selection box has three tabs. Select the second one, marked "key signature". In the upper portion of the window, you can see a preview of the key signature you are defining.

By using the scroll bar you can add sharps and flats to,or subtract them from, the current key signature. Advanced users can also define custom keys by using buttons on the right, and defining the root note in the bottom field.

The "visible key signature" check box  enables you to define whether the key signature is displayed or not. To ensure good readability of your score however, avoid setting invisible key changes.

The key signature display mode defines whether natural symbols are used to cancel the previous key signature or not.

The Propagate change till the end of tune check box enables you to transpose all key changes following the one you are setting. For example, if you switch key signature from C to D, all following key signatures will be increased by two semitones. For example, a G key signature (one sharp) will be transformed into A (three sharps).

At the bottom of the window, pop-up menus enable you to select :

• The transposition to apply to notes when a new key is inserted into a score.
Notes following this key change can be : Which staves the key signature will be applied to.
It can be the current staff only, all staves in the score, or only selected staves.
In the two last cases, you can select whether the change is made in absolute or relative mode.
In absolute mode, key change is applied as is to other staves.
In relative mode, key change is applied taking into account any key difference between the two staves. In case of doubt, select relative mode.
Absolute and relative mode

As described above, a key change can be applied absolutely or relatively.

When applied absolutely :

When applied relatively :

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