The BMW 735i uses the P21W bulb type across all listed brake light positions for the 1985-1992 model years.
The P21W is an ECE-standard single-filament bulb rated at 12 volts and 21 watts, designed for signalling applications such as brake lights.
P21W bulbs use a BA15s bayonet base, which is a single-contact bayonet fitting with a 15mm collar.
The BA15s base designation indicates a single-contact configuration, meaning the bulb carries one filament and one electrical contact at the base, distinguishing it from dual-filament variants that use a BA15d base.
Both the standard brake light and the Center High Mount Stop Light on the BMW 735i share the same P21W specification across the 1985-1992 model years.
The ECE designation on the P21W indicates compliance with United Nations Economic Commission for Europe regulations, a standard applied across European and many international vehicle markets.
Because the standard brake light and the Center High Mount Stop Light use the same bulb type, a single P21W part number covers both positions on the 735i for the full 1985-1992 range.
The 21-watt output of the P21W places it in a higher power category than common parking or position bulbs, which typically operate at 5 watts, reflecting the greater visibility requirement for brake signalling.