It doesn't matter whether one or the other terminal you connect to is grounded or not, positive or negative with respect to ground.
The reading you get on the meter will depend entirely upon the potential difference between the two leads. If the "+" lead is more positive than the "-" you'll get a plus reading. If the "-" lead is more positive than the "+" you'll get a minus reading.
You could connect the positive test lead to a point which is at +6V to ground, the negative lead to a point which is at +9V to ground. Even though both test points are positive with respect to ground, the meter will give you a reading of -3V.
If you are using a digital meter it does not matter. But look for the plus or minus symbol on the display. Then you can reverse the leads. But the voltage value will not change. I think with an analog meter your pointer will try to go negative, thus providing no reading. This is when you need to put the right meter lead on the right conductor????? Anyone else?
I think with an analog meter your pointer will try to go negative
It will. Depending upon the range selected and the applied reverse voltage it will do anything from moving slightly to the left of the zero mark to slamming hard into the left-hand stop (and possibly causing damage).
You can get center-zero movements intended to allow either polarity to be applied (the ammeter in your car is a good example).
The VTVM (vacuum tube voltmeter) or its modern transistor equivalent often has a polarity reversing switch so that negative values can be read with the cable shield left connected to a grounding point.
Man, I remember one sure fire way to cause the veins in the vocational school instructor's head to pop out was to reverse the leads on the schools analog multimeters while doing dc testing since he had repeatedly warned that it would damage the meters.
There is one, not so obvious, thing to watch out for on DVMs, VTVMs, and VOMs. Some are reversed on resistance or diode checks. All of my Flukes have been correct, or should I say intuitive, with - on the black lead for diode checks. My Sabtronics DVM at home has - on the red lead on all resistance scales. I need to keep that in mind because I often use it to check LEDs on the 100 ohm X10 scale. I try not to reverse bias LEDs because they typically have a very low reverse breakdown voltage. Joe
Most of the older, conventional-style VOMs were wired in the way that Joe describes, so that polarity from the internal battery was the reverse of what you might expect on ohms ranges. It simplified the switching to do it that way.
Black (common) lead
The Avo 8 series meters (among others) actually use the positive terminal as the common for all ranges. The negative test lead is then connected to one of three terminals for switched ranges, 2500V A.C., or 2500V D.C. (it's not "negative" as such on A.C., of course).