for the pyranoses, these depict those shapes (conformations) on which no, one, or two ring-atoms lie outside the plane of the ring. If there are two such atoms para to each other, they can lie 1) on opposite sides of the plane (trans), giving chair forms, or 2) on the same side of the plane (cis), giving boat forms. Similarly, there are six boat conformations. If the two (trans) exoplanar atoms are meta to each other, the conformation is a skew form; if the two atoms are ortho to each other, the conformation is a half-chair form. For the furanoses, the envelope conformations have one ring-atom exoplanar. If there are three adjacent, coplanar ring-atoms (the two exoplanar ring-atoms on opposite sides of the plane), the conformations are twist forms. [Sir Walter Norman Haworth]