An important feature of the indices, and of Basic, Advanced, and List searches, is the use of thesauri. The purpose of a thesaurus is to establish equivalences between different terms corresponding to the same person or thing, so that, for example, 'C moll' corresponds to 'Ut mineur': the terms are different but the meanings are the same, so normally you will want to find pieces in a given key regardless of whether the original publication—and hence the Hofmeister XIX listing—is in German, French, or another language. When you use the indices or carry out a thesaurus search, you select one thesaurus term, which automatically activates the other, equivalent terms in the original Monatsberichte. Thesauri are available for Composer, Key, Opus, Publisher, Publication place, Country, and Class, and they are automatically used for indices and for Basic search. For Advanced and List search you must use thesauri for Key and Country (otherwise you will get misleading results), but you can choose whether or not to use them for Composer, Opus, Publisher, Publication place, and Class. There are no thesauri for Title or Registration number.

There is a common interface for all thesaurus searches. When you click on the 'Thesaurus' icon, the relevant thesaurus window opens; enlarge the window if you cannot see 'Submit search' at the bottom. This window will contain a complete listing of thesaurus terms from which you can select one or more. In the smaller thesauri, use the scroll bar to find the term(s) you want; in the larger ones, first use the alphabetical index at the top, or type the first few letters into the 'Starting with' box and click on View. To select a search term, highlight it and then click on the 'Insert' button at the bottom of the window; to select multiple terms hold down the Control key, or to select a range hold down the Shift key. It is not necessary to close the thesaurus window after use; be aware however that if you leave a thesaurus window open, you cannot open another. Therefore, if it seems impossible to open a thesaurus window, check whether another is already open, and if so, close it.

Certain thesauri require additional explanation. The Opus thesaurus consists of an opus number, followed where appropriate by subdivisions within that opus number. The most convenient way to use it is to type the opus number into the 'Starting with' box, at which point only entries beginning with that number will be displayed. If necessary the process of narrowing down the field can continue (e.g. '81a', '5 No. 2'); note that some entries involve ranges, so that you might also find Op. 5 No. 2 under '5 No. 1-3'. Posthumous works are identified by 'posth'. The Key thesaurus uses a simple code according to which 'C+' means C major, and 'C-' means C minor, in each case including the equivalents in other languages. 'C' means that the mode is not specified in the Monatsbericht entry, which probably means that it is in fact C major. (It follows that if you want to search for music in either C major or minor, do not just select 'C': select C, C+, and C-. Similarly, if you want to be sure of finding all music in C major, search for C as well as C+.) In the case of Country, note that the thesaurus is based on 2006, not historical, boundaries; country information is not present in the original Monatsberichte, but has been generated by us based on the places of publication. Class refers to Hofmeister classification; this is a sufficiently complex topic to require a page of its own.

The extent to which we have grouped different terms as equivalent varies from one field to another, according to the degree of interpretation involved. In the case of key designations there is little difficulty in establishing equivalent terms in different languages. In the Publisher thesaurus we have grouped what we believe to be equivalent terms but left names followed by 'in Commission' as separate entries: this extract from the Publisher thesaurus, showing how we have grouped together forty variants of 'Breitkopf & Härtel' and two 'in Comm.', is a good illustration of why thesauri are necessary. In the the case of Publication place we have grouped terms we believe to be equivalent (e.g. 'Berlin', 'Berl'). For both Publisher and Publication place, coding limitations mean that we cannot disambiguate entries with identical names even when this information is available (thus 'Beadeker' may refer to one of several different publishers, while 'Frankfurt' can mean either Frankfurt <Main> or Frankfurt <Oder>). In creating these thesauri we have made extensive use of the Deutsches Buch- und Schriftmuseum (DBSM) website.

Because of the sheer numbers involved, it is the Composer thesaurus that presents the worst problems. Sometimes you will see separate thesaurus entries for what you feel sure are the same person, for instance, 'Bach, C.P.E.', 'Bach, C.Ph.E.', and 'Bach, Carl Philllip Emanuel'. We could have linked all of these as being the same person. But what about 'Adam, A.', 'Adam, A.C.', 'Adam, A.Ch', and 'Adam, Ad.'? There's no simple way to establish whether Monatsberichte items bearing these names are by the same person, and there's no clear dividing line between where we can and where we can't be sure of people's identity. We have therefore adopted a conservative policy: the composer thesaurus links those entries which differ in orthography (such as 'Adam, A.C.', 'Adam, A C', 'Adam, A. C.', 'Adam, (A.C.)' or 'A.C. Adam'), but not those entries where substantive interpretation or research would be needed to establish whether or not they are equivalent. This makes Hofmeister XIX less convenient to use than a modern, specifically-researched bibliographical database, but that is one of the limitations of using a nineteenth-century publisher's catalogue for a purpose it was never designed for. The inconvenience of selecting a number of thesaurus entries to search for one composer can however be minimised, though not eliminated, by using the facility to select multiple items from the thesaurus window, and the 'Back to search' button for repeated searches (which means you do not have to re-enter the same search terms each time).

The complete thesaurus listings can be downloaded from the Download page. In addition to documenting the equivalences built into Hofmeister XIX, these constitute a record of the many variant forms found in the Monatsberichte.