  Mechanical extremal materials, a class of metamaterials that exist at the
bounds of elastic theory, possess the extraordinary capability to engineer any
desired elastic behavior by harnessing mechanical zero modes -- deformation
modes that demand minimal or no elastic energy. However, the potential for
arbitrary construction and reprogramming of metamaterials remains largely
unrealized, primarily due to significant challenges in qualitatively
transforming zero modes within the confines of existing metamaterial design
frameworks. In this work, we show a method for explicitly defining and in situ
reprogramming zero modes of two-dimensional extremal materials by employing
straight-line mechanisms (SLMs) and planar symmetry, which prescribe and
coordinate the zero modes, respectively. We design, test, and reprogram
centimeter-scale isotropic, orthotropic, and chiral extremal materials by
reorienting the SLMs in place, enabling these materials to smoothly and
reversibly interpolate between extremal modalities (e.g., unimode to bimode)
and material properties (e.g., negative to positive Poisson's ratios) without
changing the metamaterial's global structure. Our methodology provides a
straightforward and explicit strategy for the design and tuning of all
varieties of two-dimensional extremal materials, enabling arbitrary and dynamic
mechanical metamaterial construction to completely cover the gamut of elastic
properties.
