Universal set
= Universal set
{title2=$U$}
A universal set $U$ fixes the objects under consideration in a particular discussion; each <set> under consideration is a <subset> of $U$. For example, $U=\{1,2,3,4,5,6\}$ makes the <complement> of $\{1,2,3\}$ equal to $\{4,5,6\}$. This use of “universal” is local to the discussion: it does not assert the existence of a <set> containing every <set>. Stating $U$ makes exclusion rules precise.