Dang it Cheryl! I knew I'd forget something... here's some more pix.
It's not a (dinner plate) "charger" if that's what you mean by an underplate, it's only 8" across. And it's deep: the well in the center is 3/4th's of an inch or there'bouts. That's where I came up w/wine coaster.
I'd love to know what this is so that............... I can use it!