 Let ≤ be a reducibility relation and let A and B be two of its degrees.Then A ≤ B if and only if there is a set "A" in A and a set "B" in B such that "A" ≤ "B".This is equivalent to the property that for every set "A" in A and every set "B" in B, "A" ≤ "B", because any two sets in "A" are equivalent and any two sets in "B" are equivalent.