key signature
US /ˈkiː ˌsɪɡ.nə.tʃər/
UK /ˈkiː ˌsɪɡ.nə.tʃər/

1.
armature, armature de clé
any of several combinations of sharps or flats on a musical staff at the beginning of a composition, indicating the key in which the composition is written
:
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The pianist checked the key signature before starting the piece.
Le pianiste a vérifié l'armature avant de commencer le morceau.
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Changing the key signature alters the pitch of many notes.
Changer l'armature modifie la hauteur de nombreuses notes.