On Doubling 3rds

If you were brought up in a classical harmonic world, you will have been taught that, whilst you may double a minor third, you should never double a major 3rd. Then you go out into the world of real music and meet doubled major 3rds in repertoire by composers you were led to believe knew what they were doing. The story kind of changes then: well, yes you can double major 3rds if you really have to, but we don’t really want you to.

It feels confusingly like doubling 3rds is one of those adult activities surrounded by double standards, like drinking or sex. Grown ups can do it, but the circumstances in which it’s okay are shrouded in mystery, and children are encouraged not even to think about it. It’s no wonder we all go off the rails in our teens, as we try to figure out how we can do these strange adult things in the absence of a clear understanding of the rules.

Now, I can’t help you with sex or alcohol, but I can get you out of your adolescent flailing around with major 3rds. There are actually clear rules as to which major 3rds you can and can’t double, and the reason you’re not told about them when you’re starting out is that the rules rely on knowledge that you usually only encounter later in the process of learning harmony. So the blanket rule makes do until the more detailed concepts come along.

The deal is this:

Never double a leading note

This is the real taboo. Some rules are made to be broken, but not this one. If you break this rule, then you will punished by your harmony sounding yucky, and there aren’t many consequences worse than that.

Now, this applies to the leading note – i.e. 3rd of the dominant chord – in the key you’re in as the clear and obvious case. But it also applies to the 3rd of every secondary dominant you use, since the point about these is to create a localised mini-tonic in the following chord. So, in pretty much any dominant-type 7th you use, the 3rd functions as a leading note and MUST NOT BE DOUBLED, on pain of unsatisfactory musical results.

In tonal music, dominant-type sevenths account for a lot of the major thirds you need, so you can see why extending the ban on doubling to major 3rds in general is an expedient guideline for the beginner.

But we do also have major triads. And these contain the 3rds that you could, if necessary, double.

So, what counts as necessary? This comes down to voice-leading. If the tonal logic and/or prosody of the individual lines leads inevitably and satisfyingly to the 3rd of a major triad in two parts, then consider the doubling. If other solutions would be awkward, or would bump the performer out of their expressive mode to manage their way round a technical corner, then it is okay to prioritise expressive flow over ideal balance of voicing. You need some net musical gain from the unorthodox doubling to offset the impact on sonority.

You may double the 3rd of a major triad, that is, but do it carefully, wisely and in service of a greater good.

Two final pieces of advice:

1. If you feel your major triad’s 3rd needs doubling, also double the root. Having a root, a 5th and two 3rds is risky, as it makes the chord harder to balance. Having two lots of root/3rd dyad is safer, as each third can bind into a root to glue the texture together.
2. Use root position – i.e. make sure that both 3rds are above rather than below their roots. Trying to balance a first inversion triad with two major 3rds in is an ungrateful activity that you don’t want to put people rehearsing and performing your music through. And by definition if you’re doubling both root and 3rd you can’t have a second inversion triad as you’ve got no 5th to give to the bass.

Now, welcome to the adult world of the responsible use of harmony.