A sixth below is a third above turned upside down. Take the harmony note a third above the tune and drop it an octave: it is now a sixth below. Same two notes, same sweetness, but the tune is on top now, and your line is underneath.

That is why altos, tenors and baritones sing it so much. When the melody is high and yours is the lower voice, the sixth below is the close harmony that fits your range.

Hear it

The upper line is the tune. Solo the lower one until you can hold it in your head, then play both and sing the lower line against the tune.

Why it is harder than the third above

Two reasons. The tune is above you, and your ear is drawn upward toward the highest line it can hear, so the pull back to the melody is stronger from underneath. And the note itself is further from the tune than a third is, five scale steps instead of two, so you cannot find it by leaning on the tune. You have to find it from the key.

The first note is the sixth of the scale when the tune starts on the tonic. In C major, the tune sings C and you sing the A below it. Sing the tonic, walk down three steps, and hold.

Practise it

Blend's sixths-below lessons start in the second stage, after the third above has settled, on short tunes with the harmony written out and the option to hear it alone. The staff comes away as you improve. Every note is graded.