A three-dimensional classification for WN stars is presented using (1) the HeII 5411/HeI 5875 ratio as a primary indicator of ionization, (2) FWHM 4686 and EW 5411 as indicators of line width and strength, and (3) an oscillating Pickering decrement as an indicator of the presence of hydrogen. All WN stars in the Galaxy and two-thirds of the LMC stars are classified on the new system. Almost all spectra inspected fall smoothly into categories within which the spectra are very similar. All ionization subclasses show a tight correlation between line strength and width, with stars containing hydrogen at the weak, narrow end, and WN/C stars near the strong, broad end. H^+/He^++ correlates with strength and width with a cut-off for the presence of hydrogen, which is slightly dependent on ionization subclass, at about FWHM 4686=30A and EW 5411=25A. The correlations found indicate that high (initial) mass stars evolve as narrow-line stars from late to early ionization subclass. Lower (initial) mass stars evolve with increasing line strength and width, probably to earlier ionization subclass. The HeII 4686/NV, III4604-40 ratio shows a clear correlation with Galactocentric radius, presumably an effect of the Z gradient. CIV 5808/HeII 5411 shows no such correlation. LMC WN stars can be classified without difficulty by the criteria established for Galactic WN stars. While individual spectra of a given subtype are similar in the two galaxies, the frequency distributions over ionization subclass, over EW and FWHM in subclasses WN4 and WN5, and hydrogen content in subclasses WN6-8 are different. The effects are presumably due to metallicity, but the causal connection is unclear.