Wikipedia
A niddy-noddy (plural niddy-noddies) is a tool used to make skeins from yarn. It consists of a central bar, with crossbars at each end, offset from each other by 90°. The central bar is generally carved to make it easier to hold. Either one of the crossbars will have a flat edge to allow the skein to slide off, or will be completely removable. Niddy-noddies can be constructed of many different materials including wood, metal, and plastic. Wood is traditional, and most quality niddy-noddies are still made of wood. Budget spinners occasionally use niddy-noddies made from PVC pipes.
Manufactured niddy-noddies can be made of different sizes, producing skeins from 12 inches in length to 4 feet in length. The most common size, however, produces a two-yard skein. Very small niddy-noddies are generally used for small samples. Many spinners will spin a sample length of yarn, ply it, and skein it using a niddy-noddy before washing. Then the spinner can see if the yarn is as desired or not.
By counting the number of wraps for the skein, and measuring the length of one wrap, the approximate yardage can be calculated. The yardage is approximate because an exact yardage requires an even tension throughout wrapping the whole skein. Also, a very large skein requires wrapping the new layers on top of the old, which increases the length of the top layers.
A niddy-noddy is held by the Christ Child in Leonardo da Vinci's Madonna of the Yarnwinder, Scottish National Gallery, Edinburgh.