Why, however, should anyone transcribing a historical source for analysis care about the existence of such endeavours? How can a knowledge of SGML help in understanding a text, in placing it into its proper context? It should be relatively uncontentious that the mechanical drudgery of transcribing, editing and reproducing texts is enormously simplified by the uncoupling of the tasks of data interpretation (tagging) and data reproduction (formatting). The former is an essentially hermeneutical and scholarly act, while the latter is not. Coombs, Renear et al have persuasively argued that the proliferation of sophisticated tools for desktop publishing have effectively seduced scholars from their true vocation (Coombs 1990) and that SGML offers a chance to regain the high ground.
Suppose that you have obtained, or created, a DTD which adequately describes the kind of source texts you wish to process. How will it be of use to you? You will use it firstly as a means of checking that each individual document you process conforms to the model you have defined. As well as providing you with a diagnostic check on the accuracy of your keyboarding, this will also provide what the Americans call a reality check on your interpretation of the sources concerned: you may be forced to confront aspects of your sources to which an initial, possibly over-hasty, assessment has blinded you. Moreover, because each part of an electronic text (or at least, each part that has been tagged) is equally accessible, equally processable, you can analyse the contents of your sources with a far higher degree of accuracy and sophistication than either a simple transcription or a database of derived observations would provide.
Historical sources do not belong to one individual however. More perhaps than other kinds of resource, they must be shareable, whether from economic or ethical considerations. An electronic text encoded in SGML, with its associated document type definition and appropriate documentation, is a permanent asset, independent of time, place and personality.