VocabularyFor some time I've been increasingly uncomfortable with the language that modern writers put into the mouths of historical characters. While I acknowledge that stories must be written in a style that readers find easy, there are times when a writer's personal vocabulary could benefit from being channelled to mark out a character's lifestyle and personal circumstances.
Georgette Heyer plundered cant and slang dictionaries for her lower-class characters. For example, in The Toll Gate her highwayman Jerry and people who associate with him have a quite different vocabulary from that of Captain Staple, who is the son of an aristocratic family. (However it's not a good idea to copy Heyer's expressions, because in the same way that modern computer programmers build-in redundant code she also invented her own colourful phrases to check how many rivals were plagiarising her work.)
Historic correspondenceOriginal letters suggest that even during the Regency and the reign of William IV, everyday polite English between people of the educated class was surprisingly like the language we write today. A glance at the correspondence between William Wordsworth and his wife Mary in the early years of the 1800s, or the letters from 1829 onward between a young lawyer, William Holt, and his fiancee Mary Cox, shows that sentences were typically long, few contractions were used (can't, won't), and the & mark often replaces and. Other than those stylistic points the language isn't noticeably archaic. Most of the time they use the formal "you" when addressing each other, and use "thee" only when writing of more intimate feelings.
Spoken languageThere's considerable difficulty for writers in researching spoken language, because recorded conversations even in works such as Mayhew's London Labour and the London Poor are filtered through the author's hand. They are probably a factual record of what was said, but they are not verbatim transcriptions. A further difficulty in composing historical conversations is that regional phrases and dialect would have been even more strongly localised than they are now, yet we don't want to make a character's dialogue totally incomprehensible to modern readers.
U and Non-UIn 1956 the terms 'U' meaning Upper Class and 'Non-U' meaning non-Upper Class appeared in Alan Ross's academic paper for the University of Birmingham: U and Non-U – An Essay in Sociological Linguistics. Nancy Mitford also wrote about these differences in 1956 in Noblesse Oblige.
There are some interesting distinctions made, notably the fact that the vocabularies of working class and upper class were similarly robust and forthright, while it was the middle class who used "delicate" phrases involving euphemisms and French words. This may sound strange, but compare the following lists. Which words will be in the vocabulary of the upper class, and which of the middle class?
Food for thoughtWhile some of the terms above are relatively modern, for historical fiction I hope the point is still clear: upper class characters shouldn't express themselves in fancy language, any more than working class ones should. The characters who do will mark themselves at once as middle class.
Good writing for an English "period" story should reinforce my belief in the society being described, and when the author writes about aristocrats in a way that is unmistakably middle-class, s/he loses my trust. This, in my observation, is a point where a good many modern writers (and especially writers who are not English-born) fall down.