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The one-dimensional WN classification defined by Smith (1968, MNRAS 138, 109), based primarily on the relative strengths of N\siii\ -- N\sv\ lines, has been described as unsuccessful because it fails to define a unique luminosity and temperature for a given spectral type (e.g. Conti \ea\ 1983, ApJ 268, 228). Part of the problem lies in the lumping of ``weak'' and ``strong'' lined stars (Hiltner \& Schild 1966, ApJ 143,770 ``A'' and ``B'' types) into the same spectral bins. Recently Hamann, Koesterke, \& Wessolowski (1993, AA 274, 397) (HKW) have determined the luminosities and temperatures for 60\% of the 71 apparently single WN stars in the updated Sixth Galactic WR Catalog of van der Hucht \ea\ (1988, AA 199, 217), from the equivalent widths (EW) of He\sii\ \lam 5412 and He\si\ \lam 5876. I use their data to plot the EW$_{\lambda 5412}$ as a function of the ratio EW$_{\lambda 5412}$ / EW$_{\lambda 5876}$. This plot shows a clear separation between ``weak'' lined and ``strong'' lined WN stars with a dividing line at EW$_{\lambda 5412}$ = --35 \AA. Both the strong and weak line sequences follow the N ionization classification along their respective curves. There is also a hint of a further bifurcation in the weak line sequence.
I argue that this is an empirical HR diagram for the WN stars because for each sequence (weak and strong); there is a linear correlation between temperature and the equivalent width ratio, and an inverse linear correlation between the absolute visual magnitude and the EW$_{\lambda 5412}$. (This second correlation is just the ``Baldwin Effect'' for WR stars; noted by Morris \ea\ 1993, ApJ 414, L25). I will discuss a similar classification system by Smith, Shara, \& Moffat (1995, IAUSymp 163, 48) who have suggested using the FWHM of He\sii\ \lam 4686 in place of EW$_{\lambda 5412}$ and an additional third dimension measuring the H/He abundance ratio. I will also discuss the use of alternate luminosity (EW$_{\lambda 4686}$) and temperature (EW$_{\lambda 4686}$ / EW$_{\lambda 10830}$) indicators.
This work has been supported by NAG5-619.
