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The Subdominant Chord: The IV Chord
The subdominant chord of the major key, the IV chord, is called "subdominant" because, even though it only contains notes from the major scale and forms the only other major triad (besides the I and V triads), the subdominant chord does not have “dominant” power to focus harmonic traffic towards the tonic, the way the V7 chord does.
As a major triad containing two notes not found in the other major triads, the subdominant chord belongs with the tonic and dominant 7th as one of the three basic chords of the major key.
The minor second chord (the IIm chord) serves as the subdominant of the relative minor key and belongs with VIm and III7 as one of the three basic chords of the minor key.
One interesting thing about the diminished chord, the VIIº chord. Because the four-note dominant seventh (V7) contains all three notes of the diminished chord (and three out of four notes of the diminished 7th chord), you can often substitute the VIIº or VIIº7 chord in place of the V7 chord to create a striking harmonic effect.