The web service <ref xmlns="http://www.tei-c.org/ns/1.0" target="http://correspSearch.net">correspSearch</ref> aggregates machine-readable letter indexes from printed or digital letter editions and makes them centrally searchable (Dumont et al 2023; <bibl xmlns="http://www.tei-c.org/ns/1.0" corresp="https://edition-humboldt.de/T2AXRQ5N">Dumont 2018</bibl>). In addition, it provides an <ref xmlns="http://www.tei-c.org/ns/1.0" target="http://correspsearch.net/de/api.html">interface</ref> that also enables automated retrieval of these data and reuse in one's own programs. Since Humboldt's correspondence<bibl xmlns="http://www.tei-c.org/ns/1.0" corresp="https://edition-humboldt.de/UGSUKML5">(Schwarz 2018</bibl>) was already considered too extensive to be edited in a complete edition in the 1960s, it has since been published scattered in individual correspondence editions or even essays (especially in the journal <ref xmlns="http://www.tei-c.org/ns/1.0" target="https://www.hin-online.de">Humboldt im Netz</ref>)<bibl xmlns="http://www.tei-c.org/ns/1.0" corresp="https://edition-humboldt.de/HKNVP8BH">(Schröder 2008</bibl>). In correspSearch, almost all of the more than 6000 <ref xmlns="http://www.tei-c.org/ns/1.0" target="https://correspsearch.net/de/suche.html?s=http://d-nb.info/gnd/118554700">published letters to and from Alexander von Humboldt</ref> have been brought together for the first time and made searchable for researchers.