Double-Subscript Notation for Voltages
I've taught it this way before to give a mental picture of double subscript notation.
\$V_{ab}\$ is the voltage measured by a voltmeter when the red (positive) lead is connected to node a and the black (negative) lead is connected to node b.
A single subscript, \$V_a \$, is measured with red to node a and black to node 0 (&quot;ground&quot;).
It then follows that:  \$V_{ab} = V_a - V_b \$


Link to reference: Voltage notation conventions