It's all Mike Flynn's fault; he posted this video first. Pomplamoose takes on a nicely-written old standard that calls for very close harmony* and what could be more close than harmonizing with yourself?
The song's only 2:20; they natter on amusingly for a couple of minutes after.
Remarkable work and a fine example of what you can do with this livin' in the future stuff! (They have recorded some more recently-written music, too).
* How close? A persistent rumor had it that Gary Troxel of The Fleetwoods enlisted to get away from constant rehearsals with his singing partners. It's not true; he was drafted. All three of them continued to perform for years after. But it was believable; close harmony takes work. Among their hits, Mr. Blue and Come Softly To Me.
4 months ago