Audio: tonality (0:04)
tonality plays tonal and atonal music on a harpsichord. The tonality figure shows the score. The first bar of music has a key, the second does not.
Tonality comprises a mix of scientific ideas about what music is and perceptual ideas about what music should be.
Tonality comprises two separate ideas, key and consonance. In an ideal world each one of these would be treated as a separate and distinct feature in its own right. This would help to distinguish the factual, has music got a key, from the perceptual, is it pleasing. In reality, the two ideas get conflated into a single term, tonality.
Tonal music has a key, atonal music does not. This is about the only thing people agree on, that tonality means music with a key.
Tonal music, in the sense of music with a key, is far more prevalent and widespread than atonal music.
The problem starts when tonality gets mixed up with consonance. Tonality often embraces ideas of what is, or should be, consonant and dissonant. In this respect, it has moved from the sphere of science to that of perception. Some people find tonal music consonant and pleasing and atonal music dissonant and displeasing. Others will tell you that not all tonal music is consonant and some is downright unpleasant. Some will also agree with the statement that atonal music is dissonant but will disagree with the statement that it is unpleasant. Tonality is two different concepts wrapped in a single word. No wonder it is problematic.
Tonality is always used throughout the guide in a factual sense to mean music with a key. It is not used to mean music that is consonant or dissonant.