CK Bulb Socket vs. Standard Bulb Socket
CK also knows as SCK, SACK, or SRCK like this: 3157CK, 3157SACK, 3157SRCK
Only a handful of vehicles made in certain years, from a few manufacturers use a CK wired socket. Sometimes different car and truck manufacturers will change the type of wiring configuration they’ll use to power certain turn signal and brake light bulbs. Normally you would have Positive and Negative terminals on either side of the bulb, but the “CK” type wiring is different.
A CK/SRCK is like the traditional sockets you see in most Japanese, and American vehicles except it’s wired like this: