The 1157 bulb is the sole bulb type specified for the White/GMC WHM across all listed front and rear turn signal positions.
The 1157 is a dual-filament bulb, meaning it contains two separate filaments within a single envelope, one rated at 21W and one at 5W.
It operates on a 12V electrical system, which is standard for automotive applications of this type.
The 1157 uses a BAY15d bayonet base, a two-pin offset base design that prevents incorrect insertion by positioning the two pins at different heights.
The 1157 conforms to ANSI standards and is recognized in both the United States and Canada, indicating a standardized specification shared across North American markets.
The White/GMC WHM uses the same 1157 bulb size for both front and rear turn signals across the 1988-1991 model years listed, meaning the same bulb type serves all four turn signal positions.
Because the BAY15d base uses offset pins, it is not interchangeable with single-pin bayonet bases such as the BA15s, despite a similar physical appearance.
The dual-filament nature of the 1157 is common in applications where a single bulb serves both a turn signal and a secondary function, such as a parking or running light, using each filament independently.