When can you double the third of a chord

Answer: Certainly you can - though as a general principle it's better to double one of the other notes if the chord is major. Best choices for doubling in a major triad in order of desirability: root, 5th, 3rd.

1. Why is doubling the third bad?
2. Why is it advised to avoid doubling the third of a chord?
3. What do you double in a 3rd inversion chord?
4. Is it better to double the third or fifth?

Why is doubling the third bad?

As mentioned in Dekkadeci's answer, doubling the third can lead to using parallel octaves (neither of which sound bad) and it can sound like a voice dropped out. This is a problem with the dominant chord; the third is note 7 of the scale and it strongly leads to the tonic.

Why is it advised to avoid doubling the third of a chord?

Avoid doubling the third of major triads particularly when in first inversion. (The overtones of the third of the chord work against the other notes. The lower the first inversion bass note of the chord is, the worse it sounds.)

What do you double in a 3rd inversion chord?

Secondary Chords: Double the third. First Inversion (Major): Double the root or fifth. First Inversion (minor): Double the third. Second Inversion: Double the fifth.

Is it better to double the third or fifth?

In practice the note most often doubled is the tonic. This reinforces the overall sound of the chord and stresses the key-center. Doubling the fifth strengthens the "stability" of the chord, and doubling the third emphasizes the major or minor aspect of the chord.