Sources
The primary source of prescriptive judgments is descriptive study.[citation needed] From the earliest attempts at prescription in classical times, grammarians have observed what is in fact usual in a prestige variety of a language and based their norms upon this. Modern prescription, for example in school textbooks, draws heavily on the results of descriptive linguistic analysis. Because prescription is generally based on description, it is very rare for a form to be prescribed which does not already exist in the language.

However, prescription also involves conscious choices, privileging some existing forms over others. Such choices are often strategic, aimed at maximising clarity and precision in language use. Sometimes they may be based on entirely subjective judgments about what constitutes good taste. Sometimes there is a conscious decision to promote the language of one class or region within a language community, and this can become politically controversial — see below.

Sometimes prescription is motivated by an ethical position, as with the prohibition of swear words. The desire to avoid language which refers too specifically to matters of sexuality or toilet hygiene may result in a sense that the words themselves are obscene. Similar is the condemnation of expletives which offend against religion, or more recently of language which is not considered politically correct.

It is sometimes claimed that in centuries past, English prescription was based on the norms of Latin grammar, but this is doubtful. Robert Lowth is frequently cited as one who did this, but in fact he specifically condemned "forcing the English under the rules of a foreign Language".[5] It is true that analogies with Latin were sometimes used as substantiating arguments, but only when the forms being thus defended were in any case the norm in the prestige form of English. A good example is the split infinitive: supporters of the construction frequently claim the old prohibition was based on a false analogy with Latin, but this seems to be a straw man argument; it is difficult to find a serious writer who ever argued against the split infinitive on the basis of such an analogy, and the earliest authority to advise against the construction, an anonymous American grammarian in 1834, gave a very clear statement basing his view on descriptive observation.[6]