Written by Gary Ewer, from “The Essential Secrets of Songwriting” website. FOLLOW GARY ON TWITTER
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If you’ve written a melody, or if you’ve got melodic ideas you want to work into your next song, your job is to invent harmonies that present that melody in the best possible light. The task may seem daunting, but there are really only two things that most chord progressions need to do: establish key, and harmonize notes. The process is actually not as tricky as you might think.
Let’s say you’ve written this melody: C-G F-E-D-F-E-C-D-C, where each note gets a beat, and the last two notes are held longer. How do we go about harmonizing this melody?
Let’s assume that we’ve chosen the half note (2-beat note) as the duration of each chord. If our melody above is made up of notes that are a beat long, that means we’ll have two notes for every chord choice. But how do we know that we’ve chosen chords that really work?
You’ll want to create progressions that do two things: 1) They make your chosen key feel like the “home” key; and 2) They properly harmonize the notes of the melody. It does no good to say, “I’m going to use the circle of fifths progression” if your melody doesn’t fit those chords.
When I create chord progressions, I concern myself first with the start of the progression: I want it to feel like C major (in this case) is “home”. And I concern myself with the end of the progression: I also want this to make C major feel like I’ve arrived back home. So I create the start in a strong kind of way, and I finish up as well with strong progressions. Then I fill in from both ends to create something that works.
I mentioned earlier that I had decided to change chords every two beats, harmonizing two melody notes with each chord. So look at each pair of notes in your melody, and choose a chord that both harmonizes the notes properly and makes C major feel like home. Most of the time you’ll want to be sure at least one of the notes of the pair fits your chosen chord, preferably (but not always) the first one.
Here are the note pairs:
[C-G] [F-E] [D-F] [E-C] D – C (notice that the last two notes don’t get paired because they are each 2-beat notes).
Given those note pairings, I’ve put a “bad” progression below, and then a good one. Each chord below harmonizes each successive pair of melody notes, with the last two chords harmonizing the final D-C of the melody:
BAD: C Em Dm C Dm C
GOOD: C F Dm C/G G7 C
The so-called “bad progression” doesn’t work well because even though each chord does the job of harmonizing the given notes, they fall down on the job when it comes to making C major feel like the home key.
The “good progression” makes C feel more like the home key because of the use of strong chord movement. The chord C moving to F is strong because those roots are a 4th apart. The Dm moving to C/G has the bass again moving a 4th, which is great, and G moving to C at the end makes C sound like the home chord.
There is so much more to be said about chords, but the basic rule, no matter how long your progressions or melodies are, is this: Your progressions should properly support the melody, and should make one chord stand out as feeling like “home.”
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For even more help and examples to show you how to harmonize a melody, download Gary Ewer’s songwriting e-books. 
I disagree on several points with your analysis, which is based on the classical aproach of keys and melodies and see chords as a secondary feature defined by the melody and the key.. Much modern music, like pop or rock, didn’t start with a melody or key, later fitting into a chord sequence, but it was merely the other way around, like finding a nice melody within a blues chord progression. Secondly, the “key” in which pop and rock are played is almost of no importance. For instance, actually there is a debate going on in which key “Hey Joe” is written. No conclusion is satisfying as this song uses five chords that mathematically can never ever fit into one key. Bach and Mozart composed within keys while they used counterpoint as their guide. There is no proof whatsoever that they had the knowledge or the symbols of chords that todays musicians have. Your explanation is a confusing mix up of classical and modern music.
Hi Jacob:
Thanks very much for writing and giving your thoughts on this. You are right that melody-first composition is more common amongst classical composers, but that are many songwriters who have used/are using a melody-first method. (One of my eBooks, ‘How to Harmonize a Melody’ is meant specifically for songwriters who have a melody and want to add chords, and I often get feedback from songwriters happy to have that book to guide them in their melody-first songwriting.)
To say that key is almost of no importance in pop and rock is just simply incorrect. Most of the time, chords work together around a tonic chord as a tonal focus. It is correct, however, to say that it’s quite possible (perhaps even common) to create chord progressions without knowing or even caring what the key is. But that’s not the same as saying that key isn’t important. As an analogy: it’s possible for someone to know how to build a garage that doesn’t fall down without knowing the physics involved in why it doesn’t fall down. That doesn’t mean that the physics is of no importance… it simply means they don’t happen to know the physics.
I’m not sure what your comment about no proof that Bach and Mozart had the knowledge or the symbols of chords that today’s musicians have is meant to convey. You chose two composers who were masters of chord theory, and how melody and chords work together. Perhaps I’m not understanding your comment there.
In conclusion, Jacob, I’d encourage you to consider that of all composers of tonal music, regardless of genre, have all been trying to do something similar: to create a musical work in which melody is supported by a coherent harmonic journey, whether their compositional process starts with the chords or ends with them. In the end, classical, pop, blues, jazz, country and folk music are often more similar than they are different.
Thanks again, Jacob, for taking the time to write, and I do appreciate your comments.
-Gary
Hi, Great article and nice explanation.. but I have a question about the starting chord that I should start the chord progression with as mentioned above.
My question is what if the melody is on the Key of C but the melody does not have the C note as the first note in the melody .. what we should do in this case?
Hi Andreas:
If the chord is C, the most common starting notes for melodies would be C, E or G. I don’t have stats on this, but I’d think that starting on C if the chord is C would actually not be most common… probably starting on E or G would happen more frequently.
Then there’s the possibility of starting on a non-chord tone. If you decide to start on either a D or an F as a starting note, it’s best to follow with a note one step lower. In other words, it sounds most solid to have a D move down to a C, and an F move down to an E.
-Gary