音階

1. Origins, structure, and basic meaning

• Structure: 音階 (おんかい) is a noun for a musical system of notes arranged in pitch order.
• Basic usage: 音階を弾く makes the ordered notes the object played on a piano, while 音階が使われている makes the bright-sounding scale the subject of a passive construction.

2. Common pitfalls and similar expressions

• Played object and employed system
  • 音階を弾く means playing an ordered scale rather than one isolated note. In 音階が使われる, the scale that a piece uses has become the subject of the passive construction.