For a two-particle (spin 1/2) system, three fundamental forms of complete sets of commuting observables (CSCO) composed of two-body operators σ1ασ2βare given. The corresponding simultaneous eigenstates constitute three types of maximally entangled states, one of which is the well known Bell basis. This paper discusses the possible pairs of spin observables whose simultaneous measurement outcomes are definitely correlated in probabilistic fashion.