A corner clamping apparatus capable of adjustably clamping two joined, mitered corner pieces to form a 90-degree corner with a single clamp. The clamping apparatus includes three rigid L-shaped supports, adjustably mounted relative to one another to accommodate differently sized corner pieces and/or variations in the corner pieces or their mitered edges. Four adjustable clamping feet retained in the rigid L serve to adjust the corner pieces to produce an exact miter joint or seam, and to hold the adjusted corner pieces until they can be more permanently joined.