**
Universal set
**
is the set which contains all the elements of the other sets.

Let A = {1, 2, 3, 4} , B = {3, 4, 5, 6}, C = {6, 7, 8}

So, universal set will have all the elements of set A, B, C

∴ U = {1, 2, 3, 4, 5, 6, 7, 8}

Note that:

All the sets are subsets of Universal set

A ⊂ U

B ⊂ U

C ⊂ U