Some terms in the Index Catalogue are spelled with æ 'ae' and œ 'oe' ligatures. Cross references from Medical Subject Headings (MeSH) spelling to Index Catalogue spelling will be made in the near future but do not currently exist. Series 1-3 consistently applies ligatures in several key areas - subject, names, and titles. Ligatures are dropped in official subject headings in Series 4 and 5. However, some articles continue to use oe and ae spellings in titles.
| Term | Series | IndexCat Instruction |
| Fœtus | 1-3 | search fœtus for subject headings |
| Fetus | 4-5 | search fetus for subject headings |