So monosaccharides can either be aldehydes or they can be ketones. Aldehyde containing monosaccharides are known as aldosis, and ketone containing monosaccharides are known as ketosis. So the smallest functional monosaccharide, as we said, in just a moment, contains an N value of three. So we have three carbon atoms, six H atoms and three oxygen atoms. So what exactly is the meaning of the D and the L? And what does the D represent?