The X‑structures and/or mechanisms examined throughout this book series constitute a unique class of scissor‑like structural or linkage systems constructed from rigid, elastic, or flexible rods. These systems typically form characteristic X‑shaped patterns arranged in half‑layer, single‑layer, or multi‑layer configurations. Their fundamental building blocks may take various geometric forms, including X‑shapes, quadrilaterals, V‑ or L‑shapes (corresponding to half of an X or quadrilateral), as well as polygonal, Z‑ or S‑shaped, W‑shaped, and A‑shaped motifs. A defining feature of these structures is their capacity to incorporate additional active or passive mechanical elements—most notably linear or nonlinear stiffness, damping, and inertial components—integrated in diverse configurations to realize customized and highly tunable dynamic behaviors, with desired nonlinearities.