The concept of borrowing a note from the parallel major or minor key is very simple. In a minor key there are three pitches that are lowered a half-step from the same scale degree in a major key. These are the 3rd, 6th, and 7th. When one or two of these scale degrees are borrowed from the parallel minor into the major key, a new chord quality appears. When a m3, m6, or m7 is imported from a minor key into its parallel major, a borrowed chord is formed.
The most commonly borrowed pitch is the m6th. Using this pitch to replace the 3rd of a subdominant triad (IV) in a major key yields a subdominant minor chord (iv). In the key of C, a diatonic IV chord is spelled F-A-C. A minor iv chord, created by borrowing the m6th, is spelled F-Ab-C. This can add interest and unexpected color to the music.
It is much less common to see a major scale degree borrowed into the parallel minor. However, a major 3rd borrowed for the tonic chord at the final cadence is frequently encountered. It is referred to as a Picardy third.
The following chart shows the quality of diatonic triads and 7th chords commonly used in both major and minor keys. Any of the minor chords might be borrowed into the parallel major. In addition to those in the chart below, a major triad built on the lowered 7th scale degree, or subtonic, is available to borrow. It’s root appears in the descending form of the melodic minor.
Given that lowered scale degrees generally resolve downward in the direction of their inflection, the minor 3rd, 6th, and 7th usually resolve to the pitch a half-step below. Altered tones are rarely doubled in four-voice harmony, with the exception of the triad built on the lowered 6th scale degree (VI) in a major key.