3-ply corrugated stock has a sheet of flute glued in between two sheets of paperboard.  This is also known as ‘single wall corrugated stock’ and is most commonly used for making cost-effective corrugated boxes and inserts.  On the other hand, 5-ply corrugated stock has two sheets of flute alternatively layered between three sheets of linerboard.  Also known as ‘double wall corrugated stock’, this version of corrugated stock is sturdier and specifically used for heavier products.