An algebra, A, on a set X satsfies * empty set in A, X in A * B in A => comp(B) in A (comp(B) = X/B) * C,B in A => C union B in A A sigma algebra, A, on a set X satisfies: * empty set in A, X in A * B in A => comp(B) in A (comp(B) = X/B) * {Ai} 1<=i Union(i=1,infiniti,Ai) in A Examples: A={empty set, X} A=borel sigma algebrea=all open sets Note that intersection is defined. A intersct B = Comp(Comp(A) union Comp(B))