.

Then, does Common Mean positive or negative?

It will almost certainly be connected to one side of the battery supply, often, but not necessarily, the negative one. So will many other components. So the negative connection of the supply in this case would be called "common" or 0V. A positive supply and a negative supply.

Likewise, what is negative DC? Negative voltage in a circuit is voltage that is more negative in polarity than the ground of the circuit. The first circuit is oriented one way and the other circuit is oriented the same way but with the voltage source flipped around, so that is has opposite polarity orientation than the first.

Regarding this, what is DC common?

"Common" is a point of reference shared throughout a circuit, typically the node in reference to which every other node is described. In the case of a single-pole double-throw (SPDT) switch, common (also called the pole) is the terminal which the "normally open" and "normally closed" terminals switch to.

Does polarity matter in DC?

Polarity results from the fact that an electrical circuit has a negative and a positive pole. Direct current (DC) flows in one direction, resulting in a constant polarity. Though AC itself has no polarity, when AC electrodes are used on DC they usually operate best on one specific polarity.