Q: Is a secondary chord an analytical label for a specific harmonic device that is prevalent in the tonal idiom of Western music beginning in the common practice period? ¶
A: Yes, and the use of diatonic functions for tonicization.
Q: Are secondary chords a? ¶
A: Yes, and "cornerstone," of popular music and jazz.
Q: Are secondary chords altered or borrowed chords? ¶
A: Yes, and chords which are not in the key.
Q: Were secondary chords not used until the Baroque period and are found more frequently and freely in the Classical period? ¶
A: Yes, and even more so in the Romantic period, and, although they began to be used less frequently with the breakdown of conventional harmony in modern classical music, secondary dominants are a "cornerstone," of popular music and jazz of the 20th century.