The Astro Tools 78311 11-Piece 3/8" Drive 1/2 Size Socket Set - 9.5 - 19.5mm is designed for undersized, swollen, or damaged hardware. Have you ever grabbed, for example, a 13mm socket and it's too tight, but a 14mm feels loose? And it's not SAE? While perfect-fitting sockets are ideal, not every bolt and nut on vehicles remains factory perfect to the millimeter over time and service life. However, this set allows the use of 0.5mm under sockets for rounded and damaged bolts and nuts, and the use of 0.5mm over sockets for swollen or corroded bolts and cap-covered nuts. For hand tools and impact wrenches, use a hammer to fit 1/2 size under sockets where needed.