Final answer:
In a major key, the iv minor chord is frequently borrowed from the parallel minor key to add expressive variety to the music. This technique is known as modal mixture and is widely used across many music genres.
Step-by-step explanation:
Within the context of Western music theory, when working in a major key, musicians often 'borrow' chords from the parallel minor to add variety and color to the harmony. One of the most frequently borrowed chords is the iv minor chord. In a major key, the usual IV chord is major, but the borrowed iv chord from the parallel minor key brings a poignant or introspective quality to the music. This concept is known as modal mixture or modal interchange, and it is commonly found in various genres, including classical, jazz, and popular music. For example, in the key of C Major, the borrowed iv chord would be F minor (F-Ab-C), which could appear in a progression where the F major chord (the IV chord) would typically be expected.