Arranging : Time, Tempo, Meter, Key, and Chords : Key Signatures : Key Signature Ruler

Key Signature Ruler
The Key Signature ruler lets you add, edit, and delete key signatures. You can use key signatures to indicate key and key changes in your Pro Tools session. Key signatures can also be used for certain diatonic functions, such as transposing in key or constraining pitches to the specified key.
 
To view the Key Signature ruler, do one of the following:
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Select View > Rulers > Key Signature.
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You can also show or hide the Key Signature Staff for the Key Signatures ruler.
To view the staff for the Key Signature ruler:
1 Show the Key Signature ruler.
2 Do one of the following:
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Select View > Rulers > Key Signature Staff.
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