The bipolar spectrum is the continuous range of depressive diseases, ranging from bipolar disorder to unipolar depression.

A simple nomenclature system was introduced in 1978, although there are others, by Angst, J., et al, to easier label individuals' affectedness within the spectrum, following a clinical study by the Psychiatric University Clinic of Zurich (PMID: 708227).

Points on the spectrum using this nomenclature are denoted using the following codes:

• 'M' severe mania
• 'D' severe depression (unipolar depression)
• 'm' less severe mania (hypomania)
• 'd' less severe depression

Thus, 'mD' represents a case with hypomania and major depression. A further distinction is sometimes made in the ordering of the letters, to represent the order of the episodes, where the patient's normal state is euthymic, interrupted by episodes of mania followed by depression ('MD') or vice versa ('DM').

On this scale, major depression would be denoted as 'D'. Unipolar mania ('M') is, depending on the authority cited, either very rare, or nonexistent with such cases actually being 'Md'.

Unipolar hypomania ('m') without accompanying depression is not observed in the medical literature. There is speculation as to whether some high-achieving individuals are actually 'm', with their successful social functioning keeping them out of sight of the mental health profession.

Although it is officially considered a personality disorder rather than an affective/mood disorder, some experts advocate adding borderline personality disorder (BPD) to the bipolar spectrum. BPD has a lot of similarities to rapid-cycling bipolar type II and other depressive disorders, and many patients show a positive response to the same types of medication.