The foundation is the unit placed beneath the mattress, sometimes referred to as the box spring (that is, in those cases when it still contains springs). A well-made foundation elevates and supports the mattress, as well as prolongs the life of your mattress. Keep in mind that mattresses and foundations are designed to work as a unit, so it is generally recommended that they be purchased together as a set.
The majority of foundations sold today are box springs, torsion module foundations, or slatted bases. A box spring is a wooden box with heavy coil springs that take some of the load off of your mattress. Torsion module foundations work similarly to box springs, but the coil springs are replaced by torsion bars (heavy wire bars typically bent at 90-degree angles). Relative to traditional box springs, torsion module foundations generally create a slightly more rigid feeling. A slatted base, also known as a built-up foundation, is made of wooden slats that span across the frame of the foundation unit. Slatted bases are (not surprisingly) firmer than box springs or torsion module foundations, and they can also be used for foam mattresses. It's important to make sure that the slats on your base are no further than 5cm apart from each other.
For those who enjoy the comfort of today's extra-thick mattresses, but prefer getting into bed without a step-ladder or a running start, many manufacturers now also offer "low-profile" foundations for certain mattress models. A low-profile foundation is an extra-thin foundation made to accompany an extra-thick mattress so that the total height of the bed will be more in keeping with a traditional sleeping surface.