For many rock types, Mohr-Coulomb (MC) failure criterion is a reasonable approximation to strength data, featuring a linear relation with two principal stresses and two material parameters that describe a stress intercept (e.g. uniaxial compression or uniform triaxial tension) and pressure sensitivity (e.g. internal friction angle φ). A criticism of MC is the absence of the intermediate principal stress. Paul-Mohr-Coulomb (PMC) criterion removes that limitation by including three principal stresses. PMC has the advantage over other multi-axial stress criteria in that three material constants, such as one stress intercept and two friction angles, one for compression φc and one for extension φe, are readily identified. PMC failure criterion is reviewed and data from a series of conventional triaxial compression and extension experiments on Indiana limestone are analyzed. The extension friction angle is larger than the compression friction angle, a sufficient but not necessary condition of the intermediate stress effect. To capture the behavior of the rock in multi-axial loading, PMC is extended to include the results of plane strain compression experiments through the construction of two planes with six parameters.