Nominal reference is very flexible---the same object may be called a dalmatian, a dog, or an animal when all are literally true. What accounts for the choices that speakers make in how they refer to objects? The addition of modifiers (e.g. "big dog") has been extensively explored in the literature, but fewer studies have explored the choice of noun, including its level of abstraction. We collected freely produced referring expressions in a multi-player reference game experiment, where we manipulated the object's context. We find that utterance choice is affected by the contextual informativeness of a description, its length and frequency, and the typicality of the object for that description. Finally, we show how these factors naturally enter into a formal model of production within the Rational Speech-Acts framework, and that the resulting model predicts our quantitative production data.