Introduced in the last quarter of the 17th century, this device improved the accuracy of balance-control timekeepers from 15 minutes per day to within two minutes per day. Early balance springs had few turns and some were even made from straight wire until it was discovered that by raising the outer turn of the spring and bending it towards the centre to form an over-coil, the centre of gravity of the spring retains greater concentricity to the axis of rotation of the balance, thereby improving the clock's accuracy.