easyDelta rigid frame by dolpin is licensed under the Creative Commons - Attribution - Share Alike license.
The 3 brackets are mounted between the towers and the top plate to create a rigid frame and a square angle between the plate and towers.
I printed these on a well tuned prusa i2 to ensure that the bracket is exactly square.
A m4 nut holder is added to insert m4 nuts into the 20mm extrusions. I had no t-nuts around, but this works just as well.