I have an outer cylinder and an inner cylinder given. They are hollow(like if you were to take a sheet of paper and tape one edge to the other), grounded, and the inner cylinder is inside the outer cylinder. Between these cylinders is an ungrounded cylinder. I am now asked to calculate the capacitance of this system. I have calculated all the charges based on the potential of the middle cylinder. I don't quite understand what is meant by the capacitance of this system though. I know that the capacitance of one conductor is the ratio of its charge to its potential, and the capacitance of two conductors would be the ratio of charge to potential difference in the sense that they each have the same charge but with different sign. But what is meant here?