Why VDD and not simply VD?
The convention of letters VAB for voltage means the potential between A and B. Voltage is a potential measured with respect to another point in the circuit. For example VBE is the voltage between base and emitter. Ground does not have a specific "letter". So the convention of repeating letters is used, like VDD or VEE to refer to the point relative to ground. Using single letters in this context adds more confusion since Vs may refer to the voltage of a source "s" (which may be different than VSS if there are multiple sources in series, etc) and not the voltage between a transistor's emitter & ground.
Even without transistors in a circuit, voltages can be referred to with the style VAB or V12 to refect potential between A and B or point 1 and point 2. Obviously order is important, since for two points in the circuit A and B, VBA = -VAB.
Bibliographic reference: "If the same letter is repeated, that means a power supply voltage: Vcc is the (positive) power-supply voltage associated with the collector, and Vee is the (negative) power-supply voltage associated with the emitter". Text abstract from Paul Horowitz and Winfield Hill (1989), The Art of Electronics (Second ed.), Cambridge University Press, ISBN 978-0-521-37095-0. Chapter 2 - Transistors, page 62, Introduction.
