Key
A fixed scale with a key signature.
What it means
A key designates the frame of reference of a piece: a root, a mode, and the resulting stock of notes. It is more than a scale — it includes the fact that a particular note acts as a centre, to which the music remains related and to which it returns.
In the notation
The stock of notes is shown by the key signature at the beginning of the line. This, however, does not name the key itself, for the same key signature applies to a major key and its relative minor. Which of the two is meant is revealed only by the music: the closing note, emphasis on the tonic, the occurrence of the leading tone. In minor this never appears in the key signature but always as an accidental within the bar.
Musical use
The ordering of keys follows the circle of fifths: every step of a fifth upward adds a sharp, every step downward a flat. On the opposite side, the two series meet in enharmonically equivalent keys.
Within a piece the key rarely stays unchanged. Tonicizations and modulations lead into related keys and back; the construction of many forms rests precisely on this change, for instance in sonata form, whose second theme stands in a different key.
Historically notable is the question of key character. In unequal temperaments, keys actually sounded different, because the intervals were of different widths; the traditional attributions rest on this. In equal temperament this difference falls away, and what remains are differences of register on the instruments.
The key results from the root and the mode. In the major-minor system there are 24 keys, which can be arranged in the circle of fifths. The accidentals at the beginning of the line indicate the key but do not fix it unambiguously, because relative keys have the same key signature; what is decisive is which note is felt as the goal.
Performance practice
Determine the key and mode at the outset and say the accidentals aloud to yourself. As you work on a piece, trace where it leaves the key and where it goes — this structures the form and carries the shaping.