When dealing with particular situations in mathematics, it is often easier to have a set which contains all the elements we deal with. This set is called a universal set and we want to define it properly.
A universal set $U$ is a set of elements fulfilling all the necessary or sufficient properties that we deal with in a particular situation.
In a Venn diagram, we draw the universal set as a frame in which we place the sets of our consideration (here a set $A$).
| | | | | created: 2018-12-09 13:16:29 | modified: 2018-12-09 13:23:28 | by: bookofproofs | references: , 
 Kohar, Richard: “Basic Discrete Mathematics, Logic, Set Theory & Probability”, World Scientific, 2016
 Reinhardt F., Soeder H.: “dtv-Atlas zur Mathematik”, Deutsche Taschenbuch Verlag, 1994, 10