The Smart Socks measure your child's heart rate, oxygen levels, and sleep behavior - from the first night at home to the first day of school. Enjoy every developmental milestone of your child with peace of mind. The package includes a total of 4 socks (2 for the right foot and 2 for the left foot) in sizes 1 and 2, which your baby can wear from 0 to 18 months old. The sensor and base station are not included. Works only with the Owlet Smart Sock 3.