Middle notes comprise the majority of the blend. They are the scents that round out the extremes of the top and base notes. Lavender and marjoram are good examples of middle notes. Lastly, add the top notes. These are the sharp, terse scents that are the first to hit the nose.
Most citrus oils are top notes, as are eucalyptus and peppermint.
Remember to label your aromatherapy blends and keep notes. Create a quiet, peaceful environment in which to do your blending.