Relays have three pins, four pins, and five pins. First explain the four stitches (30, 87; 85, 86) where 30 and 87 are disconnected and 85 and 86 are the ends of a coil. 30 connecting lines, 87 connecting electrical equipment, 85 connecting lines, 86 connecting control switches. For the three-lead type, 30 and 85 are connected on one line, because they are connected to normal fire. As for the five leads, only 87a (in the middle) is added, and 87a and 30 are connected.
Typically, four-pin relays are used as current switches. When you measure 85 86 with a multimeter, you will find that these two legs are conductive and must be in the ohm range.

The two legs of the relay are in one set, so there are two sets in total. One group is the positive and negative poles of the attracting coil, so the negative pole of the attracting coil should be grounded. You measure the two legs with resistance with a multimeter, the electromagnetic switch group shows 1, and the resistance is infinite. Grounding the negative probe of the multimeter's 12v DC voltage to ground, determining the positive is very simple. Turn on the corresponding switch and insert the positive probe into each jack of the relay socket. The device corresponding to the positive pole with voltage is the positive pole, which has the function of fuse and switch.