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 post open frame rack 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 UPGRADED SUITE: RIVECO enthusiast 18u network rack suite meets your one-stop-shop needs. Including all necessary hardware: assembly screws & nuts, leveling feet and casters and one piece of 8 outlets PDU for daily usage VARIOUS SIZES: 19.7 " (500 mm) (W), 21.7" (550 mm ) (D), 41.4"1050 mm with casters(H). There are more sizes 6U, 9U, 12U,15U ,22U,25U to choose from HEAVY -DUTY: The 18u rack features high quality SPCC cold-rolled steel and excellent work