Johnson also seems to have been the first to recognise that language is made up of multiple areas of discourse, or domains, each with its own specialised vocabulary, and each with its own specialised meanings for words, even if these words were common words of the language.

Early dictionary makers clearly thought about domains for the words they were defining since they list them on the title pages of their dictionaries. Blount's Glossographia lists the following on the title page:

Glossographia: or, A dictionary, interpreting all such hard vvords, whether Hebrew, Greeke, Latin, Italian, Spanish, French, Teutonick, Belgick, British or Saxon; as are now used in our refined English tongue. Also the terms of divinity, law, physick, mathematicks, heraldry, anatomy, war, musick, architecture; and of several other arts and sciences explicated.

Blount's address 'To the Reader' makes it clear that the function of his dictionary is to explicate terms which would be useful to the young gentleman: so, terms of heraldry are included along with 'such and so many of the most useful Law-Terms as I thought necessary for every Gentleman of Estate to understand', and he lists the terms of science he has included as deriving from 'Logick, Astrology, Geometry, Musick, Architecture, Navigation, &c. with those of our most ingenious Arts and Exercises, as Printing, Painting, Jewelling, Riding, Hunting, Hawking, &c.' These are clearly the pursuits of the young gentleman, so the breadth of lexis included in his dictionary is restricted by what would be useful to such a reader.

Blount's demarcation of domains in the prefatory material is typical of other dictionaries, even if their range of domains is different, but, in common with other early lexicographers, he does not carry this demarcation through into the body of the dictionary. As a practising barrister and the author of a law dictionary himself, the one domain one might expect him to mark in the text is that of law terms, but he does not clearly demarcate the legal sense of a term from its common meaning. The most he does is to imply the distinction as in this entry for 'arrest':

Arrest (Fr.) in the common signification it is well known for a seisure of, or Execution served upon a mans person or goods. But we sometimes use it (as the French) for a Sentence, Decree, Order or final Judgement of a Court.

Contrast this with Johnson's entry for the same word:

1. In law. ...An arrest is a certain restraint of a man's person, depriving him of his own will, and binding it to become obedient to the will of the law, and may be called the beginning of imprisonment.
2. Any caption, seizure of the person.
3. A stop.

Here the legal term is clearly set apart from other common meanings of the word and it is given primacy in the entry.

Johnson's practice is to state explicitly, usually in square brackets before the definition, the domain to which a particular sense belongs. This practice is not universally applied, but he does mark off major domains such as law terms with careful thoroughness.

This is yet another departure from his aims as stated in the 1747 Plan. There he had thought that he would omit the 'terms of particular professions since, with the arts to which they relate, they are generally derived from other nations, and are very often the same in the languages of this part of the world'. Perhaps Johnson had law terms particularly in mind since these mostly derive from law French. However, when it came to compiling his dictionary Johnson perhaps realised that it would be impossible to ignore such a large and important area of the language and he includes a great number of specialised technical terms, many of them taken directly from technical dictionaries such as John Harris's Lexicon Technicum.