The 20th century saw dramatic innovations in musical forms and styles. Composers and songwriters explored new forms and sounds that challenged the previously accepted rules of music of earlier periods, such as the use of altered chords and extended chords in 1940s-era Bebop jazz. The development of powerful, loud guitar amplifiers and sound reinforcement systems in the 1960s and 1970s permitted bands to hold large concerts where even those with the least expensive tickets could hear the show. Composers and songwriters experimented with new musical styles, such as genre fusions (e.g., the late 1960s fusion of jazz and rock music to create jazz fusion). As well, composers and musicians used new electric, electronic, and digital instruments and musical devices.