Summary: We explicitly give , the Teichmüller space of four-holed spheres (which we call pieces) in trace coordinates, as well as its modular group and a fundamental domain for the action of this group on which is its moduli space. As a consequence, we see that on any hyperbolic Riemann surface, two closed geodesics of lengths smaller than intersect at most once; two closed geodesics of lengths smaller than are both non-dividing or intersect at most once.