For building small cabinets and tables 

Design is based on 4mm screws head sunken. 

Types

50mm x 50mm x 50mm, Corner brackets for connecting 3 shelves, 9 screws in total 

50mm x 50mm Corner bracket,6 screws

50mm x 30mm Corner bracket,6 screws

45mm x 45mm Corner bracket,6 screws

For my own purpose, I used wood shelves 18mm thick, 160mm wide and 25 mm screws.

Added extra peace for connecting 2 shelves strait with 25mm space between.