The 2 hole - gusseted inside corner bracket is a strong, external fastening method that creates a 90 degree connection. It requires no machining services and can provide infinite positioning along the profile T-slot, allowing a large degree of adjustability. The shape of the gusset provides extra material, strengthening the connection. It is a fastening method that can be repositioned and reused.
For technical details and images, please search for the product using the 80/20 interactive catalogue found here: https://catalogs.8020.net/80-20-Catalog-26/C/.
