key 

The key element represents a key signature. Both traditional and non-traditional key signatures are supported. The optional number attribute refers to staff numbers. If absent, the key signature applies to all staves in the part.

Element Information

Model

((cancel{0,1} , fifths , mode{0,1}) | (key-step , key-alter)) , key-octave*
Children: cancel, fifths, key-alter, key-octave, key-step, mode

Attributes

QName Type Fixed Default Use Inheritable Annotation
color color optional
default-x tenths optional
default-y tenths optional
font-family comma-separated-text optional
font-size font-size optional
font-style font-style optional
font-weight font-weight optional
number staff-number optional
print-object yes-no optional
relative-x tenths optional
relative-y tenths optional

Source 

<xs:element name="key" type="key" minOccurs="0" maxOccurs="unbounded">
  <xs:annotation>
    <xs:documentation>The key element represents a key signature. Both traditional and non-traditional key signatures are supported. The optional number attribute refers to staff numbers. If absent, the key signature applies to all staves in the part.</xs:documentation>
  </xs:annotation>
</xs:element>

Sample