In most cases, tri-ply bottom cookware means just that - three ply material is used on the bottom of the cookware. Let's look at a pan and check it out. An aluminum pan is usually one ply. It's all aluminum. It's a good conductor of heat, and it works well. Some pans are copper on the bottom but have a stainless steel body (and interior). The copper and stainless make a two-ply construction. The copper conducts heat well (several tens of times better than stainless), and the stainless makes for an inert and long-wearing cooking surface. In try-ply, we usually see a stainless pan, a fat copper "disk" on the bottom, and another layer of stainless over the copper (which "seals it in" there). These pans are all stainless on the outside and inside. Stainless steel is extremely tough and resistant to corrosion. They wear forever. But stainless isn't a good conductor of heat. That little disk of copper sandwiched in between the stainless layers on the tri-ply bottom makes the pan heat more quickly and cook more evenly.