  The theoretical description of the anomalous properties of the pseudogap
phase in the underdoped region of the cuprate phase diagram lags behind the
progress in spectroscopic and other experiments. A phenomenological ansatz,
based on analogies to the approach to Mott localization at weak coupling in
lower dimensional systems, has been proposed by Yang, Rice and Zhang [Phys.
Rev. B 73 (2006),174501]. This ansatz has had success in describing a range of
experiments. The motivation underlying this ansatz is described and the
comparisons to experiment are reviewed. Implications for a more microscopic
theory are discussed together with the relation to theories that start directly
from microscopic strongly coupled Hamiltonians.
