Our 18" x 18" x 12" Heavy Duty Corrugated Boxes are super strong and heavy duty 275# Double Wall/ECT-48 kraft corrugated carton with twice the protection of a standard carton. ECT stands for "edge crush test".The Heavy Duty Double Wall 275#/ECT-48 Cartons are sold in bundle quantities and ship flat to save on storage space and shipping. 18 x 18 x 18 Heavy Duty Double Wall boxes with heavy-duty construction provides greater protection and is perfect to use when extra stacking strength is required. Designed to protect your valuable items, fragile, or heaviest items.