This corner clip was designed to assemble a Corsi-Rosenthal clean air filter. I use (8) in total, 4 on the bottom, 4 on the top and these fit (4) 20x20x1 filters in a box shape then we would place our box fan on top. Third Image is not my design, but a good example of what a Corsi-Rosenthal set-up if. We used these during covid to filter air, but I have found them useful for my shop for fume and particle extraction from 3d printing.