In 1929, Haworth proposed a six-membered ring structure for sugar called pyranose, because pyran contains a ring of five carbons and one oxygen. Haworth projection formulae are shown in Figure 2.5. Haworth also called sugars containing a five-membered ring as furanose, because furan contains a ring of four carbons and one oxygen. Only 1% of monosaccharides contain five or more carbons in the open chain. In general, monosaccharides in the solution will be in the pyranose and furanose ring forms.