 The extraordinary axis is parallel to the optic axis.For a light wave normally incident upon the plate, the polarization component along the ordinary axis travels through the crystal with a speed "v"o = "c"/"n"o, while the polarization component along the extraordinary axis travels with a speed "v"e = "c"/"n"e. This leads to a phase difference between the two components as they exit the crystal.When "n"e < "n"o, as in calcite, the extraordinary axis is called the "fast axis" and the ordinary axis is called the "slow axis".