We identify least-perimeter unit-area tilings of the plane by convex
pentagons, namely tilings by Cairo and Prismatic pentagons, find infinitely
many, and prove that they minimize perimeter among tilings by convex polygons
with at most five sides.
We provide very general symmetrization theorems in arbitrary dimension and
codimension, in products, warped products, and certain fiber bundles such as
lens spaces, including Steiner, Schwarz, and spherical symmetrization and
admitting density.