The HS1 is a new range halogen bulb used in many smaller motorcycles, scooters, and snowmobiles. The only difference between the HS1 and an H4 bulb is that while the H4 has what is called a P43t fitting or base, the HS1 has a PX43t base (also written PX 43t and PX 43T. This different notation indicates that the middle pin at the base of the PX43t lamp is 1-2mm thicker than the corresponding pin of a P43t base. This thicker middle pin prevents the HS1 bulb from fitting into an automobile light socket (though the reverse is still possible). The HS1 has become the standard bulb for new two-wheeled vehicles in the United States, though any socket on such a vehicle which can support an HS1 bulb can also accept an older-styled H4 bulb. Attention for converting into H4 might be danger on both wires and lens cause melting.