The fischer projection formulas are the most convenient forms for writing the structures of sugars that show stereoisomerism. Various sugars show a property of optical rotation when observed in freshly prepared solutions changing with time. Monosaccharides in solution undergo cyclization when treated with equivalent amounts of alcohol and form hemiacetals or hemiketals. This reaction is the basis for glycoside formation by carbohydrates. Intramolecular  Hemiacetal formation results in a ring form which is either a five – membered ring with one oxygen atom, or a six – membered ring with one oxygen atom. Aldohexoses form a six membered ring, called pyranose, whereas ketoses occur as five –membered ring compounds, called furanose

An aldose may exist as a five membered ring also. The cyclic forms of sugars are written as hexagons and pentagons and these are called Haworth projections. Biologically important sugars exists in cyclic forms which are convenient to draw.