Corrugated boxes are made of corrugated board, a composite material with paperboard layers that are themselves made of different materials. In single-wall corrugated board, the central corrugated layer (called the medium) is sandwiched between two pieces of linerboard. The medium is often made from what paper manufacturers call a semichemical stock, while the linerboard is typically made using the kraft process, which produces a stronger board. Both the medium and the linerboard can contain varying proportions of recycled content.