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There are different ways of finding the volts across a component. Here on the right we have a meter which is placed across the midde resistor directly.
Another way is to place the meter as shown to the left of that middle resistor. We can then subtract one reading from the other and get the same result: 2 volts here.
Using this second method can sometimes be useful because the (black) meter lead can be kept at earth zero as a reference throughout. Keeping that reference comes in handy if we wish to see how the volts are changing on each side of the component, since both sides are referenced against the same place: earth zero.
That earth reference method is especially useful where we have a capacitor. That is because the voltages either side of the capacitor have a more complex relationship to that of a resistor.
With a resistor, the relationship is V= iR
But with a capacitor, it is not an R, it is a C
and V= iC does not work.... We need a different formula to find out what happens with C
Note also: if we were to measure the volts across the bottom resistor instead of the middle resistor, then the earth zero is already placed there for us as the reference. If however, we wish to measure the top resistor volts instead, then we could place our red meter lead anywhere between that top resistor and the next resistor, then subtract the reading from the 12v input... All of this can be seen above.
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