  Active galactic nuclei (AGN) are characterized by similar broad emission
lines properties at all luminosities ($10^{39}$-$10^{47}$ erg s$^{-1}$). What
produces this similarity over a vast range of $10^8$ in luminosity?
Photoionization is inevitably associated with momentum transfer to the
photoionized gas. Yet, most of the photoionized gas in the Broad Line Region
(BLR) follows Keplerian orbits, which suggests that the BLR originates from gas
with a large enough column for gravity to dominate. The photoionized surface
layer of the gas must develop a pressure gradient due to the incident radiation
force. We present solutions for the structure of such a hydrostatic
photoionized gas layer in the BLR. The gas is stratified, with a low-density
highly-ionized surface layer, a density rise inwards, and a uniform-density
cooler inner region, where the gas pressure reaches the incident radiation
pressure. This radiation pressure confinement (RPC) of the photoionized layer
leads to a universal ionization parameter $U\sim 0.1$ in the inner photoionized
layer, independent of luminosity and distance. Thus, RPC appears to explain the
universality of the BLR properties in AGN. We present predictions for the BLR
emission per unit covering factor, as a function of distance from the ionizing
source, for a range of ionizing continuum slopes and gas metallicity. The
predicted mean strength of most lines (excluding H$\beta$), and their different
average-emission radii, are consistent with the available observations.
