Draw a circle on a sheet of paper. Draw another circle overlapping the first one, to show two circles with a shared overlapped area.
Add a variable to each circle. For example, in the left circle, you will write "X," and in the right circle, you will write "Y" to give you a left set of "X" and a right set of "Y."
Write "X U Y" in the shared area where the two circles overlap; this defines the missing set as everything within both circles, which in this case are the variables "X" and "Y."
Add more variables to either circle and adjust the shared area accordingly. For example, if you added "Z" to the left circle and "Q" to the right circle, the shared area would then read "X U Z U Y U Q" to indicate that "X," "Z," "Y" and "Q" are found throughout both circles