Introduced in 1955, the Square D "QO" line of miniature 3/4-inch circuit breakers may be their best-known product line, used in the electrical distribution boards of many residences in North America. The "QO" designation stands for "Qwik-Open"; Square D claims these are the fastest-opening breakers in the industry, responding within 1/60th of a second, or just one full power cycle on a 60 Hz AC line. QO breakers feature a window with a visual trip indication flag, making it easy to identify a tripped breaker in a panel. A "QOB" version is also offered, which stands for "Qwik-Open Bolted", featuring a screw that secures the breaker in the panel. This is useful in industrial installations where vibration might cause a QO snap-in breaker to loosen and prevents them from accidentally being popped out while an adjacent breaker is being changed. A QO breaker could easily be clipped into a QOB panel. The QO and QOB series breakers are by far their most popular commercial products.