More specific terms would include saucer cones (1, 2), disc cones (1, 2), and flat cones (1 – on the label it says marking discs, 2), if you wish to distinguish them from safety cones or traffic cones.
Most generically, they would be called lapels (the left lapel and the right lapel). And, to the best of my knowledge, it's absolutely irrelevant whether it's a coat or a bathrobe that you're talking about—they're still called lapels. There are different types of lapels such as notch lapels, peak lapels and so on and so forth, but they're all lapels.
Best Answer
I would simply call it the underside of the table.
(Collins Dictionary)