  The linear amplification mechanisms leading to streamwise-constant
large-scale structures in laminar and turbulent channel flows are considered. A
key feature of the analysis is that the Orr--Sommerfeld and Squire operators
are each considered separately. Physically this corresponds to considering two
separate processes: (i) the response of wall-normal velocity fluctuations to
external forcing; and (ii) the response of streamwise velocity fluctuations to
wall-normal velocity fluctuations. In this way we exploit the fact that, for
streamwise-constant fluctuations, the dynamics governing the wall-normal
velocity are independent of the mean velocity profile (and therefore the mean
shear). The analysis is performed for both plane Couette flow and plane
Poiseuille flow; and for each we consider linear amplification mechanisms about
both the laminar and turbulent mean velocity profiles. The analysis reveals two
things. First, that the most amplified structures (with a spanwise spacing of
approximately $4h$, where $h$ is the channel half height) are to a great extent
encoded in the Orr-Sommerfeld operator alone, thus helping to explain their
prevalence. Second---and consistent with numerical and experimental
observations---that Couette flow is significantly more efficient than
Poiseuille flow in leveraging wall-normal velocity fluctuations to produce
large-scale streamwise streaks.
