Complex sentence: Proto-circuit parties in the late 1970s, the precursors of what later became circuit parties, were called disco parties; they only lasted one evening and were held in various large venues in metropolitan areas with large gay populations.
Split into simpler sentences: Proto-circuit parties in the late 1970s, the precursors of what later became circuit parties, were called disco parties. They only lasted one evening and were held in various large venues in metropolitan areas with large gay populations.