Question: How do we know which chords to use in Deck the Halls in key of C?
Albert's reply: I've created a simple arrangement and have made the "Deck the Halls" sheet music available for free download as a small gift to key-notes readers. (Just click the link to download!)
The rules for harmonizing a melody take years to master, but I've come up with simple rules that will allow you to get it right most of the time. Professional classical musicians all need to study 16th- and 17th-century counterpoint as well as write harmony using strict rules. It's a lot like playing chess: At any given time there are a limited number of possibilities, and in those possibilities lie the challenge as well as the beauty.
Here are some very simple rules. First, notice that all the degrees of the major scale can be harmonized using the I (tonic), IV (subdominant) and V (dominant) chords. These are the most common chords. In C major, the I chord covers scale degrees 1, 3 and 5; IV covers 4, 6 and 1, and V covers 5, 7 an 2. Adding them up, all scale degrees from 1 to 7 are included in at least one of these chords.
"Deck the Halls" starts on scale degree 5. This means that it can be harmonized using either a I or V chord. Since it's the beginning of the song, let's use the tonic (I) in order to establish the key of C major:
The second half of the phrase starts on D (in C major), or the second scale degree. If we limit ourselves to I, IV and V chords, we need to use V (the dominant) since it's the only chord that includes scale degree 2. The same goes for the B (scale degree 7, the leading tone), the penultimate note of the phrase.
We can then use the tonic (I) for the other melody notes in the second half of the phrase:
The same goes for "Don we now": It's based on scale degree 2, which means we can use the dominant (V). The next measure resolves to the tonic (I):
Download the "Deck the Halls" sheet music above to discover the remaining chords!