Is there a convention for the polarity of voltage specified as \$V_{XY}\$?
I was always taught that \$V_{XY}\$ would be the voltage of X relative to Y, and \$V_{YX}\$ would be the Voltage of Y relative to X.  So as I was taught, the answer is yes, this is always the convention.



Here is what Wikipedia has to say in "Double subscript notation":


  In electronics, the notation is usually used to indicate the direction of current or voltage ...


and later:


  \$V_{CB}\$  means the "V" from C to B. In this case, V would probably stand for voltage, C would probably denote the collector end of a transistor, and B would denote the base end of the same transistor. This is the same as saying "the voltage drop from C to B".


They cite as a reference:


  ^ Moses E. Zarudi, Shenkman, Arieh L. Shenkman (1998), Circuit Analysis for Power Engineering Handbook, Springer, ISBN 0-412-83180-5


Another reference: "Basic Electrical Engineering" By Kulshreshtha, p398

In terms of measuring with a meter, it is nicely described in All About Circuits: Kirchhoff's Voltage Law (KVL)





I now see that this question was also addressed here: Double-Subscript Notation for Voltages