Sinuous springs, sometimes called zigzag or S springs, are lengths of heavy gauge wire bent into a continuous S shape and stretched front to back across the sofa frame, clipped at intervals to keep them in place. They flex under weight to give the seat base its bounce and support.
They are simpler and less costly to manufacture than pocket springs, and when made from good quality, well gauged wire and spaced closely, they perform well in mid range and everyday sofas. Lower quality versions with thin wire or wide spacing can feel less supportive and wear faster. See our buying guide for how to check the spring system before buying.