[1] J. CZYZOWICZ A. PELC, I. RIVAL (1987) : Drawing orders with few slopes. to appear. MR 1059347

[2] J. CZYZOWICZ A. PELC I. RIVAL, J. URRUTIA (1987) : Crooked diagrams with few slopes. to appear. MR 1101772

[3] I. FARY (1948) : On the straight line representation of planar graphs. Acta Sci. Math (Szeged) 11, 229-233. MR 0026311

[4] J. HOPCROFT, R. TARJAN (1974) : Efficient planarity testing. J. Assoc. Comp. Mach. 21, 549-568. MR 0359387

[5] D. KELLY (1987) : Fundamentals of planar ordered sets. Discrete Math. 63, 197-216. MR 0885497

[6] D. KELLY, I. RIVAL (1975) : Planar lattices. Canad. J. Math., 27, 636-665. MR 0382086

[7] S. MICALI, V. V. VAZIRANI (1980) : An $O\sqrt{n\ldot |E|} algorithm for finding maximum matchings in general graphs. In: 21st Ann. Symp. Foundations Comp. Science, IEEE, Syracuse, pp. 17-27.

[8] R. NОWAKОWSKI I. RIVAL, J. URRUTIA (1987) : Lattice graphs contained in planar graphs are planar. to appear.

[9] A. PELC, I. RIVAL (1987) : Оrders with level diagrams. to appear.

[10] C. PLATT (1976) : Planar lattices and planar graphs. J. Combin. Theory B, 21, 30-39. MR 0429672

[11] K. WAGNER (1936) : Bemerkungen zum Vierfarbenproblem. Jber. Deutsch. Math. Verein., 46, 26-32.