x_{1} (shape/form) is (idealized as) a quadrilateral/two-dimensional closed shape bounded by exactly four distinct non-colinear line-segments (or circular arcs), with vertices x_{2} (ordered cyclical list) and with additional properties x_{3}.

Bounded but not necessarily convex; the reference set of x_{1} includes a simple hourglass diagram (two triangles touching at a vertex such that two sides of each are mutually colinear).