In practice and analysis, neighboring tones are sometimes differentiated depending upon whether or not they are lower or higher than the chord tones surrounding them. A neighboring tone that is a step higher than the surrounding chord tones is called an "upper neighboring tone" or an "upper auxiliary note" while a neighboring tone that is a step lower than the surrounding chord tones is a "lower neighboring tone" or "lower auxiliary note". However, following Heinrich Schenker's usage in "Free Composition", some authors reserve the term "neighbor note" to the lower neighbor a half step below the main note.