Key signature

A group of sharps or flats placed at the beginning of a musical staff to show which note letters are altered throughout a piece or section.

Accidental (music): A musical symbol that raises, lowers, or cancels a note’s pitch outside the standing key signature. An accidental temporarily overrides or supplements the signature’s usual alterations.
Musical staff: A set of five horizontal lines and four spaces used to represent musical pitches in written notation. A key signature is positioned on the staff before the notes it governs.
Transposition (music): The process of moving a passage or composition to a different pitch level while preserving its interval structure. Transposing a passage often requires changing its key signature as well as its notes.
Open key signature: A notation convention indicating a mode or pitch collection without specifying a conventional major or minor key signature. It contrasts with signatures that encode a standard major or minor key through sharps or flats.
Circle of fifths: A diagram arranging the twelve pitch classes by successive perfect fifths, commonly used to show relationships among keys. Moving around the circle reveals how signatures gain or lose one sharp or flat.
Chromatic scale: A scale containing all twelve pitch classes within an octave in Western equal-tempered music. Signatures select recurring pitches from the larger set of chromatic possibilities.
Modulation (music): A change from one tonal center or key to another within a musical composition. A new signature can mark a modulation when the music establishes another key.
Atonality: A musical approach that avoids treating one pitch as the governing tonal center. Atonal music often lacks the tonal hierarchy that conventional key signatures summarize.
Major scale: A seven-note scale whose intervals follow the pattern whole, whole, half, whole, whole, whole, half. Major signatures encode the pitch alterations needed to form a major scale from its tonic.
Enharmonic equivalence: The relationship between differently named notes that sound at the same pitch in a given tuning system. Some major and minor keys sound alike but require different signature spellings.

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