Siro spinning, also known as paired-twist spinning, was formerly referred to in China as A/B yarn and has more recently been officially named Siro spinning. In the Siro spinning process, two rovings are fed into the spinning frame at a fixed distance from each other. After drafting, the two fiber strands are delivered by the front rollers. Due to twist transmission, each strand receives a small amount of twist. The two strands are then combined and further twisted together to form a yarn similar to a plied yarn, which is finally wound onto a bobbin.

Siro spinning was originally designed for wool spinning. Its main characteristics include low hairiness, high strength, and good abrasion resistance, allowing wool yarn to be woven as a single yarn and helping produce lighter and thinner wool fabrics. However, there is still some distance to go before truly achieving weaving with single wool yarn without sizing. Later, the wool spinning industry largely moved away from this method, while it became more widely used for blends such as T/C and C.V.C. After dyeing, these yarns can create a twisted, plied-yarn-like appearance and provide a pleasant hand feel, making them popular. In recent years, due to increasing concerns about yarn hairiness, Siro spinning has also been used for fibers that are prone to hairiness, such as rayon, Modal, Tencel, soybean fiber, and even pure cotton. The raw material grade used for Siro yarn can be lower than that required for conventional ring spinning, while fabrics made from Siro yarn have less hairiness, a softer hand feel, better abrasion resistance, and good breathability compared with conventional ring-spun plied-yarn fabrics.