BOTH ROUND & SQUARE HOLES: One side round holes beams are for sound equipment mounting with 10-32 screws, while the other side square holes beams are for rack shelves and 19 network equipment mounting with M5 & M6 screws STURDY TOP& BOTTOM PLATES: Half cabinet half rack designing. 4 posts open frame with reinforced top& bottom vented plates, a good balance between air ventilation and firm structure, is convenient to put your laptop, laser projector, game console, CD player, or even a beautiful plant on the rack