Just sitting at the sushi bar in Origami is a lesson in grace and courtesy, as well as in how to taste the most delicate and sublime gifts from our seas. Each of these classically trained, articulate chefs understands that his duty is not merely to prepare and serve elegant traditional Nigiri sushi and whimsical rolls, but to illuminate his art. Ask and they will tell you why ground daikon and chili paste complement halibut or why gizzard shad is marinated; true masters, the chefs love to improvise, playing with the freshest flavors and colors in town. Origami Downtown, in the lovely Old Market Hotel, is as elegant, spare, and enlightened as haiku. Occupying two floors, with sushi bar and dining tables downstairs, full bar and service upstairs, the place can get crowded and the lines frustratingly long on a weekend night. Reservations are granted only for parties of 10 or more (otherwise, first come, first served). But the service is brisk and warm, and the bar, with its list of Scotches and sake, requires a little extra time to explore. The suburban location features bolder rolls and easier seating, and the fish is just as fine.