Early Quakers adopted Plain Speech and Plain Dress to indicate their belief in equality and integrity.  Although both are still in use, neither are commonly used.  Plain Speech included thees and thous because in the 17th century, “you” was only used in its plural form or as a way to denote the speaker was socially inferior to the listener. Plain Speech also refers to speakly without flourish as was also commonplace in earlier centuries.  Plain Dress refers to the gray clothing and lack of embellishment common among 17th to 19th century Quakers. See Quaker Jane for a thorough discussion of one woman’s decision to be Plain as well as how to adopt a Plain wardrobe yourself.