Questions relating systems powered by a 12 volt supply. This may include automotive systems and lead-acid batteries that are nominally 12 volts although be aware that under many conditions the actual voltage may be several volts higher.
12v is a common voltage supply rail used in Automotive, Computing, Battery, and Consumer devices. 12v is the highest commonly found in consumer device power supplies, though 5v (through usb power supplies) has become very ubiquitous. Other common voltages are 9v (mainly in audio devices due to the use of dual rail op amps), 3.3v (Pushed by the need for lower powered microcontroller and circuits), and 3v (mainly from using two 1.5v batteries).
Higher voltages can be found in dedicated chargers, for example, Laptops or Printers, but these are often internally regulated down to 12v, 5v, 3.3v, or lower.
Circuits connected to the so-called "12 VDC" power available in automobiles (often not anywhere close to 12 V) should be tagged automotive instead. They require extra components and special techniques for dealing with load dumps, etc. that are unnecessary circuits powered by the "nice" 12 VDC supplied by computer power supply units (PSUs) plugged into mains or most other sources of 12 VDC.