Roman 2D (Two Diamonds)
A Roman 2 opening bid shows any 4-4-4-1 or 5-4-4-0 distribution with 17-20 HCP. Responder is expected to bid his best (longest) suit; with little preference, he bids 2. With a hand interested in playing a game (or slam) contract, responder can bid 2NT, artificial. This artificial 2NT response is on in competition.
After hearing a simple suit response, opener bids the next-highest suit if responder happened to bid his short suit. Responder is then expected to correct, if necessary, to a better fit.
After hearing a 2NT artificial response, opener bids his singleton/void suit at the three-level. Responder's rebids are natural.