Complex sentence: Many of the songs in the Great American Songbook were composed for musicals, and some originally included an introductory "sectional verse": a musical introduction that typically has a free musical structure, speech-like rhythms, and rubato delivery.
Split into simpler sentences: Many of the songs in the Great American Songbook were composed for musicals, and some originally included an introductory sectional verse. The sectional verse is a musical introduction that typically has a free musical structure, speech-like rhythms, and rubato delivery.