The attorney-client privilege started in Roman times, developed in England, and came to America with the English common law. Each state has adopted attorney-client privilege protection – but memorializes it in different places. Some courts continue to apply the common law, while others have adopted statutory privilege protection.
In Coneal v. American Commerce Insurance Co., the court explained that "Kentucky Rule of Evidence 503 supplies us with the attorney-client privilege for claims under Kentucky law." Case No. 5:18-CV-00095-TBR-LLK, 2019 U.S. Dist. LEXIS 160696, at *12 (W.D. Ky. Sept. 20, 2019).
In contrast to the work product doctrine that rests on court rules (sometimes supplemented by a murky common law parallel), the attorney-client privilege can come from one or more of several sources – depending on the state.