D2S/D2R/D2C are almost the same.
D4S/D4R/D4C are almost the same.
D2 and D4 use the same base, but they are not cross compatible.
"S" bulbs are used in shielded (i.e. projector) housings (not mirror housings).
D2S type HID bulb
The "R" bulb is for the mirror housing (note the painted area of the glass).
D2R HID bulbs
The base of the "C" bulbs is compatible with either, but the glass bulbs don't have a painted area like the "R" bulbs.
Compare D2S, D2R, D2C, D4S, D4R, D4C bases
D1S and D3S bulbs look similar but are not compatible**
The D1 and D3 have an igniter in the base, while the D2 and D4 rely on the igniter inside their accompanying ballast.
The biggest difference between D1/D2 and D3/D4 bulbs is that these new D3 and D4 bulbs do not use mercury and require less energy to turn on! The easiest way to tell the newer mercury-free bulbs apart from the old ones is to look at the part where the harness is connected and you'll see it's green.
D3C HID Bulb
Few more things you should know:
You cannot replace D3 bulbs with D1 bulbs (and vice versa)! They are physically fit, but when you turn it on, the bulb, ballast, or both can be damaged.
You cannot replace D4 bulbs with D2 bulbs (and vice versa)! They are physically fit, but when you turn it on, the bulb, ballast, or both can be damaged.
D1 and D2 xenon bulbs use mercury. D3 and D4 Xenon bulbs do not.
Philips and other automotive xenon lighting OEMs have nothing but 4300k bulbs. Any other color Philips or Sylvania bulbs are fake.
