Turing degree
The Turing degrees have a partial order ≤ defined so that ["X"] ≤ ["Y"] if and only if "X" ≤T "Y". There is a unique Turing degree containing all the computable sets, and this degree is less than every other degree. It is denoted 0 (zero) because it is the least element of the poset formula_1. (It is common to use boldface notation for Turing degrees, in order to distinguish them from sets. When no confusion can occur, such as with ["X"], the boldface is not necessary.)