The BMW 850i uses the P21W bulb for both the brake light and the Center High Mount Stop Light across the 1993-1997 model years.
The P21W is an ECE-standardized bulb, meaning it conforms to European and international automotive lighting regulations rather than SAE standards commonly associated with North American markets.
P21W bulbs operate at 12 volts and consume 21 watts, which is the standard wattage for single-filament signalling applications in European-specification vehicles.
The P21W uses a BA15s bayonet base, a single-contact fitting where the "s" denotes a single pin offset to one side, which prevents incorrect installation of polarity-sensitive replacements.
Because the P21W is a single-filament bulb, it serves dedicated functions only, such as braking, rather than combined stop and tail duties, which would require a dual-filament type.
The BA15s base of the P21W is not interchangeable with the BA15d base, which carries two offset pins for dual-filament bulbs. Substituting a BA15d bulb in a BA15s socket is not physically compatible.
The uniform use of the P21W across all listed positions on the 850i means a single bulb type covers both the standard brake lights and the Center High Mount Stop Light for the entire 1993-1997 production span.