Page "Monosaccharide" Paragraph 6

If the carbonyl is at position 1 ( that is, n or m is zero ), the molecule begins with an formyl group H ( C = O )-, and is technically an aldehyde.

In that case, the compound is termed an aldose.

Otherwise, the molecule has a keto group, a carbonyl -( C = O )- between two carbons ; then it is formally a ketone, and is termed a ketose.

Ketoses of biological interest usually have the carbonyl at position 2.