The term of invariant, "that which is constant, fixed and stable" is a concept used particularly in mathematics, physics and linguistics.

The advent of equal temperament at the end of the Baroque period introduced this concept of invariance into musical composition, especially for composers after J. S. Bach. The constancy of temperament that permitted the development of a formal organization for the tonal system, established the foundations of a language based on scales with stable invariable...

The invariants: Four Clearings

• First clearing: The chorale in tears
• Second clearing: Anaphora
• Third clearing: The note's scheming
• Fourth clearing: Invariant
• Fifth clearing: The chorale in tears 2 (transcription)