The Off-Broadway reference article from the English Wikipedia on 24-Apr-2004
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Off-Broadway

Off-Broadway theater refers to stage plays or musical theater performed in New York City, in venues of a smaller size than Broadway theaters. Generally Off-Broadway shows are less expensive, less publicized, less well-known and feature less-famous performers. The smaller scale also often allows more experimental, challenging work to be presented.

The classification of theaters is governed by language in Actors' Equity contracts, rather than by whether the theater has a Broadway address.

It is not unknown for successful Off-Broadway shows to later have a Broadway run - for instance, the musicals Rent and Hair were both initially shown Off-Broadway.