In a discussion all sets are cosidered to be the subset of big set . Such a big set of which all other sets under discussion are subsets is called the universal set. We normally use the symbol U for the universal sets.

Examples: A set of all  students of a  in a school could be a universal set, if  we consider  the different sets of students whose  age is below 10 ,  equal to10 or below or equal to 12 and above 12.

The set of all numbers below 1000 is universal set , if we consider the sets  of numbers divisibe  by  2 and 3  and neither by 2 nor by 3 within 1000.