| Term | Explanation | Examples |
|---|---|---|
| 2I-N1 | 2-mestnyj Istrebitel, Napier-Lion 1 dvigatel, Two-seat fighter with one Napier-Lion engine Such designations appear until mid-30s, but are not common. |
2I-N1 |
| 2U-B3 | 2-mestnyj Uchebnij, BMW-3 dvigatel, Two-seat trainer with BMW-III engine; | 2U-B3 |