Final answer:
Regional theatres in the United States were the source of most new plays until the early 1950s.
Step-by-step explanation:
Until the early 1950s, most new plays written in the United States originated in regional theatres. Regional theatres, also known as Repertory Theatres, are located all over the U.S. and have their own resident companies of actors and creatives. They produce their own season of shows and often have multiple performance spaces to accommodate different productions.
Regional theatres play an important role in the development of new plays and as a testing ground for shows before they transfer to Broadway. Several shows that premiered in regional theatres have gone on to successful runs on Broadway, including Moulin Rouge!, Kinky Boots, Memphis, and Come From Away.