  Whereas complementarity manifests itself via two incompatible observables,
quantum contextuality can only be revealed via the joint measurements among at
least three observables. By incorporating unsharp measurements and joint
measurements into a realistic model, we reestablish an inequality due to Liang,
Spekkens, and Wiseman rigorously based on the assumption of noncontextuality
alone. Its violation therefore unambiguously pinpoints the quantum
contextuality of a two-level system. The maximal violation is attained by three
triplewise jointly measurable observables that are pairwise jointly measured in
an incompatible way. We also present the necessary and sufficient condition of
triplewise joint measurability of three unbiased observables of a qubit.
