  Nonlinear stripe patterns occur in many different systems, from the small
scales of biological cells to geological scales as cloud patterns. They all
share the universal property of being stable at different wavenumbers $q$,
i.e., they are multistable. The stable wavenumber range of the stripe patterns,
which is limited by the Eckhaus- and zigzag instabilities even in finite
systems for several boundary conditions, increases with decreasing system size.
This enlargement comes about because suppressing degrees of freedom from the
two instabilities goes along with the system reduction, and the enlargement
depends on the boundary conditions, as we show analytically and numerically
with the generic Swift-Hohenberg (SH) model and the universal
Newell-Whitehead-Segel equation. We also describe how, in very small system
sizes, any periodic pattern that emerges from the basic state is simultaneously
stable in certain parameter ranges, which is especially important for Turing
pattern in cells. In addition, we explain why below a certain system width
stripe pattern behave quasi-one-dimensional in two-dimensional systems.
Furthermore, we show with numerical simulations of the SH model in medium-sized
rectangular domains how unstable stripe patterns evolve via the zigzag
instability differently into stable patterns for different combinations of
boundary conditions.
