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.