
die Tonart
key
In music, the tonal center of a piece, determined by a tonic and a mode (major or minor). The key establishes the harmonic foundation.
Das Lied steht in der Tonart G-Dur.
The song is in the key of G major.
key / tone
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key
In music, the tonal center of a piece, determined by a tonic and a mode (major or minor). The key establishes the harmonic foundation.
Das Lied steht in der Tonart G-Dur.
The song is in the key of G major.

tone
In a figurative sense, the way something is said; the tone of voice or mood of an utterance.
Die scharfe Tonart seiner Antwort hat mich überrascht.
The sharp tone of his answer surprised me.
The word is made up of 'der Ton' (the tone/sound) and 'die Art' (the kind/type). So it literally means 'the kind of tone', which fits both the musical structure and the tone of an utterance.
Die fröhliche Tonart des Liedes passte nicht zu seiner traurigen Stimmung.
The cheerful key of the song did not match his sad mood.
You can recognize the musical meaning from words like 'Lied' (song), 'Stück' (piece), 'Dur' (major), or 'Moll' (minor). The figurative meaning refers to the manner of communication and often appears with words like 'Stimme' (voice), 'Antwort' (answer), or 'Gespräch' (conversation).
In welcher Tonart ist dieses klassische Stück geschrieben?
What key is this classical piece written in?
In a musical context, the preposition 'in' is almost always used to indicate the key. A piece of music 'steht in' (is in) a certain key.
Der Pianist begann, das Stück in einer anderen Tonart zu spielen.
The pianist began to play the piece in a different key.
When referring to tone of voice, 'Tonart' (tone) is often combined with descriptive adjectives such as 'scharf' (sharp), 'freundlich' (friendly), 'bestimmt' (firm), 'ironisch' (ironic), or 'befehlend' (commanding).
Er wechselte in eine freundlichere Tonart, als er merkte, dass er zu streng klang.
He switched to a friendlier tone when he realized that he sounded too strict.
tone / clay
Ton and Tonart are both used to describe music, but they do not mean the same thing. A Ton is a single sound, whereas the Tonart establishes the tonal center and thus the harmonic foundation of a piece. Use Ton for a single sound and Tonart for the musical foundation.

Das Klavierstück steht am Anfang in einer ruhigen Moll-Tonart.
At the beginning, the piano piece is in a calm minor key.
Das Klavier spielt am Anfang einen einzelnen tiefen Ton.
At the beginning, the piano plays a single low tone.