Any open Riemann surface R 0 of finite genus g can be conformally embedded into a closed Riemann surface of the same genus, that is, R 0 is realized as a subdomain of a closed Riemann surface of genus g. We are concerned with the set M(R 0 ) of such closed Riemann surfaces. We formulate the problem in the Teichmüller space setting to investigate geometric properties of M(R 0 ). We show, among other things, that M(R 0 ) is a closed Lipschitz domain homeomorphic to a closed ball provided that R 0 is nonanalytically finite. MSC2020: 30Fxx, 32G15 Theorem 4. No Ioffe rays of M(R 0 ) hit M(R 0 ) again after departure. For every S ∈ T g \ M(R 0 ) there exists exactly one Ioffe ray of M(R 0 ) passing through S.Thus the complement T g \ M(R 0 ) is swept out by Ioffe rays of R 0 , plays a crucial role in proving Theorem 1. Moreover, Theorem 4 allows us to call M(R 0 ) close-to-convex.