— June 1997 —
| Series | Abbreviation | Literature Coverage |
|---|---|---|
| Main Work | H | up to 1909 |
| 1st Supplement | E1 | 1910-1919 |
| 2nd Supplement | E2 | 1920-1929 |
| 3rd Supplement | E3 | 1930-1939 |
| 4th Supplement | E4 | 1950-1959 |
| 5th Supplement | E5 | 1960-1979 |
Volume Indexes: E3 and E4; if compound found in appropriate volume, references back to earlier Supplements and Main Work are given.