The acronym for a car’s center high-mounted stop lamp is CHMSL.
The CHMSL is the elevated brake light commonly called the third brake light. It sits above the vehicle’s conventional left and right stop lamps, often in the rear window area, deck lid, spoiler or roof edge. Its height helps following drivers see a braking signal when other vehicles block the lower lamps.
The United States and Canada required CHMSLs on new passenger cars beginning with the 1986 model year; the requirement later expanded to light trucks and vans. Australia and New Zealand followed in 1990, while European vehicles covered by UN Regulation 48 gained the requirement from 1998.
CHMSL is not the same as FMVSS, which is a set of U.S. vehicle-safety standards, or DRL, meaning daytime running lamp. Unlike a turn signal, the CHMSL normally illuminates steadily when the brake pedal is applied. Early studies found large reductions in rear-end collisions, while later research found a smaller but continuing safety benefit after the feature became common.