IC power-supply pin

[1] In practice VCC and VEE then refer to the positive and negative supply lines respectively in common NPN circuits.

[5] This led to VD and VS being created by supply voltages designated VDD and VSS in the more common circuit configurations.

More advanced circuits often have pins carrying voltage levels for more specialized functions, and these are generally labeled with some abbreviation of their purpose.

For example, VUSB for the supply delivered to a USB device (nominally 5 V), VBAT for a battery, or Vref for the reference voltage for an analog-to-digital converter.

High-security cryptographic devices and other secure systems sometimes require separate power supplies for their unencrypted and encrypted (red/black) subsystems to prevent leakage of sensitive plaintext.

These naming conventions were part of a bigger picture, where, to continue with bipolar-transistor examples, although the FET remains entirely analogous, DC or bias currents into or out of each terminal may be written IC, IE, and IB.

Lower-case letters and subscripts are used to refer to these signal levels at the terminals, either peak-to-peak or RMS as required.

Therefore, we see VP, VK, and VG referring to plate (or anode outside of the U.S.), cathode (note K, not C) and grid voltages in analyses of vacuum triode, tetrode, and pentode circuits.