This 21x28 is one of our LOST SIZES, which is an non-traditional size in the very popular 4:3 Cinema. Sierra is our #12 heavy weight cotton duck canvas finished at 14.6 oz., sized and triple primed with archival quality acrylic gesso, producing a ground impervious to paints leaching through and harming the fabric. Suitable for most all paint types, like oils, acrylics, and alkyds. With it's heavier weight, it tends to be stronger and slightly more uniform than other heavy weight canvases in it's class. And it's ideal for heavy paint or impasto. There is a 5/16" gap between the canvas and frame, eliminating the unwanted lines that occur when the gap is less. We never use finger joints in our wood frames, which often fail under tension and over time - the glue joints give up, and are unsuitable for paintings intended to last for centuries. Patented keyable cross braces allow the canvas frame to be tensioned in the middle, where the tension is needed the most! The Stretcher Bars are slotted in the center to accept the cross braces. Due to our unique design, the cross braces may be removed, if desired. Both corner keys and brace keys are pre-installed. Stretched using our proprietary MuseumWrap method, producing a drum-tight-stretch, up to 50% tighter than other methods, and with balanced tension across the surface. The corners are uncut and neatly folded, making it possible to remove and re-stretch the canvas on the same size frame.