A song’s underlying chords sets the psychological tone for the lyrics. Sing a "happy" word on "sad" chord, and you get irony that can help color the written lyric and add expressive depth to your story. So, a strategic application of harmony can bring a powerful dimension to how your song comes across.

One way to look at harmony is as a journey. The tonic chord (C in the key of C major) is the most stable one, so when we’re there, we feel solid and comfortable—like we’re “home.” Any chord other than that I will feel like we’re “away” from home, with varying degrees of urgency to get back depending on how many of the root’s chord tones are in the new chord.

So, to understand how far we’re stepping away, first consider the notes of that root chord: the notes C E G in the key of C major. The VI and III chords are short steps away; just one note changes (A C E and E G B, respectively), so while we’re clearly somewhere on our journey, they’ll feel like an initial, close step away, more than a distant move resulting in an urgent need to return. If we change two notes, such as to get the IV (F A C) and V (G B D), we arriving at a more distant land, which may drive us with a stronger need to return to the root. And a II (D F A) or VII (B D F) chord is clearly alien territory. Particularly the leading tone B natural wants to resolve to the tonic, C. And of course, you can step outside the key and grab a non-diatonic chord for a stronger motion, like F minor 7 or Ab major in the key of C. That will shake things up quite a bit more.

Now, there are no "laws" about which chord must lead other chords. At least, none in Massachusetts. The point is more about realizing how dramatic the shifts are, and how that fits your specific circumstance.

If things are getting a little dull harmonically, you might decide to balance it with a bolder move.

Phrase endings are particularly conspicuous inflection points in the song’s story. And so, these specific points of harmonic motion, called cadences, should be strategically considered.

Here are some common cadences. How they are named and classified varies depending on different traditions of music theory. As writer Jimmy Kachulis discusses in his book Songwriter’s Workshop: Harmony (Berklee Press, 2004, edited by yours truly), ultimately, these variations articulate the different relationships between our sense of home and being away, at some varying level of journey. Matching these nuances with your lyric story will add a strong psychological underscore to your song’s meaning and effect.

Here’s a roundup of some of the most important ones. In addition to simply ending harmonic phrases, they work well as repeated riffs.

Common Cadences

• Authentic Cadence: Any form of V to I, such as V7 to I (G7 C, in the key of C)
• Perfect Authentic Cadence: In root position, V to I (G C)
• Plagal Cadence: IV to I. (F to C, sing “A-men….”)
• Half Cadence: Anything that ends on a V chord, such as II to V (D G)
• Deceptive Cadence: Anything where the V goes somewhere other than I, such as V VI or V bVI (G A, or G Bb)

Some Others:

• Back Door Cadence: IVmi7 bVII7 I
• Full Jazz Cadence: IImi7 V7 I
• Jazz half cadence. IImi7 V7
• Incomplete Subdominant Cadence. Anything that ends on IVMa7 or IImi7
• Mixed Cadence. Preceding the perfect authentic formula with a IV, as in IV V I or IV I6/4 V I
• Phrygian Half Cadence. IVmi V (with IVmi in first inversion)

Cadence Practice

A useful songwriting exercise is to practice each cadence as a repeating riff, singing a simple lyric on them, like “I’m going home,” or “Come back to me.” Then change the cadence, tweak the melody to fit it as necessary, and consider how the lyric meaning changes. Also consider how the effect of the harmony changes when you place it different song sections: chorus, verse, prechorus. If you play guitar, About.com's Dan Cross has an excellent online guitar chord library that is a great resource for exercises like this.