Made of good grade cardboard (not flimsy or cheap.)

Very strong cardboard boxes

The clever assembly creates a no-seam bottom, that is 3 layers of sturdy cardboard

The thick cardboard makes them strong and stable.

These banker's boxes are made of sturdy cardboard.

My only complaints, which may simply be related to these being their "moving" line instead of the more "archival storage" line is that the cardboard seems a little flimsier, and the manufacturing is a bit more imprecise, with edges and corners not always lining up perfectly, though still plenty well to use to move

The cardboard is not as stiff as it could be, perhaps due to humidity (it was delivered in the rain and left outdoors by the carrier)?

They are perfect for books, and small heavy items -- sturdy cardboard, good handles, and stack well

Easy to put together with strong cardboard

Good sturdy cardboard, easy to assemble.

Easy assembly, sturdy cardboard, plus a decent size

Also, the staples brand has a stronger cardboard.

They are made of strong heavy duty cardboard

The sturdy cardboard sides allowed me stack them 4 high, without any issue

The cardboard is not as sturdy as I would have liked