Everbilt Plain Steel Speed Nut

Not all nuts are hexagonal. You can find these mechanical fasteners in other shapes, including that of the letter U. Known as U-nuts, they are commonly used in conjunction with sheet metal. Rather than welding two sheet metal panels together, you can use a U-nut.
What Are U-Nuts?
Also known as U-nut clips, U-nuts are a type of clip-on nut that resembles the shape of the letter U. They consist of a metal clip with a hole in the center. You can clip a U-nut onto the edge of a sheet metal panel.    

U-Nuts vs J-Nuts
U-nuts aren’t the same as J-nuts. They are both classified as clip-on nuts, and they both feature barbs or “legs.” The legs are the clipping appendages found on the side. The difference is that U-nuts have uniform legs of the same length, whereas J-nuts have legs of different sizes. This nuance results in a different shape: U-nuts are shaped like the letter U, and J-nuts are shaped like the letter J.