A small black triangle fills the upper left corner. (tate.org.uk)
The thick, black triangle sticks out and rises above the surface, and the piece sits atop sea - green colored clay on a block of white wood. (artefuse.com)
You can see the impact of this purchase - gift in the spectacular, Ellsworth Kelly Multi-Panel Paintings gallery that exhibits a grouping including Yellow Relief with Black (1993), Red on Red (2001), Black Triangle with White (1976), Red Curves (1996), and two 1968 paintings. (adobeairstream.com)