The elementary formula of a simple monosaccharide is CHO, where the integer n is at least 3 and rarely greater than 7. Simple monosaccharides may be named generically based on the number of carbon atoms n: trioses, tetroses, pentoses, hexoses, etc.
Every simple monosaccharide has an acyclic (open chain) form, which can be written as ; that is, a straight chain of carbon atoms, one of which is a carbonyl group, all the others bearing a hydrogen -H and a hydroxyl -OH each, with one extra hydrogen at either end. The carbons of the chain are conventionally numbered from 1 to n, starting from the end which is closest to the carbonyl.
If the carbonyl is at the very beginning of the chain (carbon 1), the monosaccharide is said to be an aldose, otherwise it is a ketose. These names can be combined with the chain length prefix, as in aldohexose or ketopentose. Most ketoses found in nature have the carbonyl in position 2; when that is not the case, one uses a numeric prefix to indicate the carbonyl's position. Thus for example, aldohexose means H(C=O)(CHOH)H, ketopentose means H(CHOH)(C=O)(CHOH)H, and 3-ketopentose means H(CHOH)(C=O)(CHOH)H.
An alternative nomenclature uses the suffix -ose only for aldoses, and -ulose for ketoses. The position of the carbonyl (when it is not 1 or 2) is indicated by a numerical infix. For example, hexose in this nomenclature means H(C=O)(CHOH)H, pentulose means H(CHOH)(C=O)(CHOH)H, and hexa-3-ulose means H(CHOH)(C=O)(CHOH)H.