made these to help assemble 2x4 in a 90 degree angle. 

Use the side holes to hold in place the 2x4s then you can screw the end together via the openings (frame with 2 holes in the corners) or from outside (frame that is open for both 2x4s). 

This came in helpful while I was alone building a 90 degree frame for a screen opening. this made it easy to hold the pieces together while I screwed everything in place.