Let's deal with "common" first.
Suppose that a car manufacturer makes one model which is available in three different colours. They are red, yellow and black, but the seats and internal trim are grey in all three. We can say that grey seats are common to them all.
For another example, a city has some streets with benches placed on their footpaths/sidewalks, some streets with public telephones at regular intervals and some streets with news-stands, but all the streets have grass verges. We can say that grass verges are common to all streets.
This might sound trivial to you, but it is important to understand what common means. It's something shared by all.
Now, let's consider "ground".
What do you mean when you use the word "ground"?
Or don't you know what it means?
Don't be shy or embarrassed. Many, perhaps most, people don't undertand it. They use it "because everybody else uses it". And, of course, if they use words they don't understand, it follows that they don't know what they're talking about.
I think you should now read my dissertation on "ground" and then come back here.
Is that any more clear? I hope so.
So, when people say "ground" and nothing more, they might mean a connection to the planet on which we live, i.e. Earth, or they might mean "chassis", or ..... they might mean something else.
Have a look, an imaginary look will do, at an AA cell. It's just lying on your hand or on a table, or might still be in the card along with another and hanging on the supermarket hook.
It has two points of connection, the positive and the negative - often called 'poles'.
Some people will refer to them as positive and "ground". The question must be asked, it really must; WHY?
Why? I'll tell you why. It's because they are behaving like sheep. They use words and expressions, words and expressions which they don't understand, "because everyone else does". Without thinking. Without understanding.
Ok, now let's consider 'negative'.
We know that a d.c. source, be it a cell, a battery or a power supply, has two points of connection, a positive pole and a negative pole.
We need to know which is which because some things we connect to a d.c. supply won't work correctly if they are connected the wrong way round. Apart from that there shouldn't be any need to say much more, if anything, about it.
Now, let's mix 'em all up.
Some items of equipment need one d.c. source.
An iPod running off its internal 3.6 volt battery. The battery has a positive pole and a negative pole. Which one is "ground"? Neither of them. It's just a silly, inappropriate and totally unnecessary word. Is its supply positive or negative? An irrelevant question. Its supply is 3.6Vdc.
It will have a jack into which you plug the 'phones. The jack will have three connections. We'll call them 1, 2 and 3. The left channel signal will be between, say, wires 1 and 2, while the right channel signal is between wires 3 and 2. Wire 2 is common to both channels. It will almost certainly be connected to one side of the battery supply, often, but not necessarily, the negative one. So will many other components. So the negative connection of the supply in this case would be called "common" or 0V. 0V because all voltages within the circuit are measured from that common point, which also happens to be the supply's negative pole. Its supply is, therefore, 3.6V positive with respect to the common connection.
I have in front of me an audio amplifier. It has a connector for the input signal and a connector, a pair of connectors actually, for a loudspeaker. One side of the input connector and one side of the loudspeaker connection/s are connected to the amplifier's common line. This amplifier needs not only one supply with reference to the common line, but two. A positive supply and a negative supply. It could be from two batteries but is actually from a mains powered circuit.
The power supply circuit has three connections from its output. A common wire, connected to the amplifier's common line, a wire which carries a positive voltage with respect to the common line, and a wire which carries a negative voltage with respect to the common. They're both 15 volts.
So the amplifier has a 15 volts positive supply, measured from the common (the 0V line), and a 15 volts negative supply, measured from 0V. We can use "0V" and "common" interchangeably. And, between the positive 15V line and the negative 15 volts line we can measure 30 volts.
Many (I suspect most) people would have been talking about the ubiquitous "ground" by now. Not me, thank you.
Now, for reasons of safety it might be considered necessary, or at least advisable, to connect any conductive parts of this amplifier (the input connector, one of the 'speaker connections and the metal chassis, to Earth. And of course "everybody" shouts "ground". Very well, if, and only if, they say "earth ground". Otherwise it's ambiguous.
If they have to disambiguate it (and they DO) why not simply say earth?
Something is only "grounded", or eathed, if it has an electrical connection to the planet Earth. But you are, I hope, beginning to realise that the widespread and ignorant mis-use of the word "ground" doesn't, on its own, tell us much - other than the fact that the user is imprecise with their choice of, and probably understanding of, words.
The circuitry of s ome equipment, my oscilloscope for instance, needs many different d.c. voltages to work. They are all derived from a mains powered supply . I can't remember what they are in my 'scope, but it isn't important. Let's just say that it needs supplies of +5, +12, +50, -12, -300 and -1,200 volts.
The following will be connected together and, along with one side of the input signal, will be called common and certainly be connected to earth:
the neg side of the +5, the neg side of the +12, the neg side of the +50, the pos side of the -12, the pos side of the -300 and the pos side of the -1,200 volt supplies.
I wonder what I have overlooked .......
Hey, here's a 1.5V battery. Touch one of the ends, go on, it doesn't matter which one. Will you get a shock, be electrocuted? No.
Here's another one. I made it by connecting a thousand 6V batteries together. So it's a 6,000 volt battery. Touch one of the ends, go on, it doesn't matter which one. Will you get a shock, be electrocuted? No.
To get a shock you have to be connected to both ends at the same time.
Connect the negative end of the 6,000V battery to a metal plate and stand on it. What will happen if you touch the positive pole of the battery while standing on the plate? Your friends will read your obituary in tomorrow's newspaper.
Now connect the positive pole of the 6,000V battery to a metal plate and stand on it. What will happen if you touch the negative pole of the battery while standing on the plate? Your friends will read .... etc.
It doesn't matter in either of these two scenarios whether the metal plate is connected to earth or not. It's irrelevant. The important fact is that you would be in contact with both the positive and the negative poles of a 6,000 volt supply.
Ok, a variation. Instead of connecting one of the poles of the battery to a metal plate, connect one of the poles of the battery to earth. It doesn't matter which one. Now, touch the other battery pole. It is probable that you will not be isolated/insulated properly from earth. There will usually be a partially conductive path between you and earth, wherever you stand. You might get killed, you might not. It depends upon the conductivity of whatever you stand on.
The danger, or otherwise, of a negative voltage is, it seems, totally misunderstood.
It doesn't matter what its polarity is. If it is big enough it will kill you.