Measuring voltage difference

Consider the following design:

To measure the voltage across R2, place a voltage probe as shown below:

Tap the  button and place the reference probe as shown below:

Run an interactive simulation to get the following results. Note that the voltage across R2 is 4.0000V. 

If you do not place a reference probe, the voltage will be measured with respect to ground, and PR1 will read 8.0000V for the above circuit.