The Vorontsov-Velyaminov scheme is a system for classifying planetary nebulae according to their visual appearance. It recognizes the following main types:
1. Stellar image (I).
2. Smooth disk (II) (a, brighter toward center; b, uniform brightness; c, traces of a ring structure).
3. Irregular disk (III) (a, very irregular brightness distribution; b, traces of ring structure).
4. Ring structure (IV).
5. Irregular form, similar to a diffuse nebula (V).
6. Anomalous form (VI).
More complex structures are characterized by combinations such as "4+2" (ring and disk), or "4+4" (two rings). The scheme was devised by the Russian astrophysicist Boris Vorontsov-Velyaminov (1904–1994).