Storage boxes offers up to 50 percent more stacking strength than standard string-and-button closure boxes Made of sturdy corrugated fiberboard. Strong, durable & secure for stacking & storing different types of inactive files. Maximum security boxes with string & button closure keep contents secure, even if box is overturned. Storage boxes offer easy one-step FastFold setup. Built for heavy daily usage & maximum stackability. Ideal for stacking without shelving.