We contend that quantitative measurements of nuclear narrow emission line strength can strongly constrain models that unify AGNs. The reasons for the importance of narrow-line luminosity LNLR are:

1. a) The lines normally arise via photoionisation by the integrated UV/soft X-ray luminosity LPHOT of the central source. Thus LNLR is directly linked to a physical quantity intimately connected with the central engine but not observable from the ground. For constant covering factor we expect an approximate proportionality between LNLR and LPHOT; this has been confirmed observationally for AGNs by estimating LPHOT from either optical non-stellar luminosity or effective ionisation parameter.

2. b) NLRs are far enough from the photoionising source to avoid the excessive obscuration that appears able to attenuate broad-line and continuum emission. Narrow-lines are radiated isotropically unlike, eg, the radio core which may be Doppler boosted. Their variability timescale of 103–4 years is intermediate between those of LPHOT and any large-scale radio emission.