Borrowed chords

Beside the six basic chord functions we can find others. The most frequently used ones: bVII, bIII, bVI, iv. In the key of C-Major these chords are Bb, Eb, Ab, Gm respectively. Note that these chords are native in the key of c-minor where their function is VII, III, VI, iv respectively. They say these chords are borrowed to the C-Major key from its parallel minor key.
Another frequent borrowed chord is II (D chord in context of C Major key). The chord can be threated as being borrwed from one of the the neigbour keys: G-Major. The bVII chord (Bb chord in context of C Major key) also can be threated as being borrowed from the neighbour key of F-Major. It's a matter of interpretation. Sometimes it really sounds like a brief change of key, for example in Drowse (bVI > bVII > I)