Why Do Some Dominoes Have A Metal Spinner?
At one time dominoes were make of two discrete materials (often ebony and ivory), and the "spinner" was actually a small pin whose purpose was to hold the white ivory face to the dark ebony side of the domino. In other words, the purpose was purely mechanical.
Today, the spinner is still used in some domino sets, largely because out of respect for tradition, but also because the spinner can help protect the face of the dominoes as they are shuffled, and can make it easier to spin or rotate the domino in place. The pin is also sometimes known as a "pivot".
The spinner is located in the middle of a domino's dividing bar, and is generally made of metal and rounded off so as to avoid scratching the table top.
Spinner pins are only used in western-style domino sets. They are not used in Chinese dominoes, since the spinner prevents the tiles from being stacked cleanly.