  Theories of spiral structure traditionally separate into tight-winding
Lin-Shu spiral density waves and the swing-amplified material patterns of
Goldreich & Lynden-Bell and Julian & Toomre. In this paper we consolidate these
two types of spirals into a unified description, treating density waves beyond
the tight-winding limit, in the regime of shearing and non-steady open spirals.
This 'shearing wave' scenario novelly captures swing amplification that enables
structure formation above conventional Q thresholds. However, it also
highlights the fundamental role of spiral forcing on the amplification process
in general, whether the wave is shearing or not. Thus it captures resonant and
non-resonant mode growth through the donkey effect described by Lynden-Bell &
Kalnajs and, critically, the cessation of growth when donkey behavior is no
longer permitted. Our calculations predict growth exclusive to trailing spirals
above the Jeans length, the prominence of spirals across a range of
orientations that increases with decreasing arm multiplicity, and a critical
orientation where growth is fastest that is the same for both modes and
material patterns. Predicted structures are consistent with highly regular,
high-multiplicity gaseous spur features and long filaments spaced close to the
Jeans scale in spirals and bars. Applied to stellar disks, conditions favor low
multiplicity (m<5) open trailing spirals with pitch angles in the observed
range $10 deg$<$i_p$<$50 deg$. The results of this work serve as a basis for
describing spirals as a unified class of transient waves, abundantly stimulated
but narrowly selected for growth depending on local conditions.
