I have two questions.
If I were to use diachronic corpora to investigate the frequency of usage for words within a word class of English that is mostly archaic today, how do I determine if the archaic words were ever considered to be a part of everyday English?
In other words, if the pronominal adverbs "therefore" and "whereby" show up in an Ngram Viewer analysis as having a 1/100ths percentile of usage, but other pronominal adverbs that typically only appear today as legal jargon have a 1/1000ths percentile of usage, how can I determine if they were ever used in everyday English?
My second question is if it is possible to determine with accuracy when is the frequency of usage of a term so low, that it is not considered to be apart of everyday language?
Thank you in advance.