What are the characteristics of free jazz?

Free jazz is the type of jazz music which developed during 1950s and 1960s. It was invented and played by the musicians who weren't satisfied with earlier styles such as bebop, hard bop and modal jazz, and who wanted to brake their boundaries and create new, free approach to music. Their aim was to extend, or break down jazz convention, often by discarding fixed chord changes or tempos, and they often turned to collective improvisation. Although, free jazz is hard to be defined, because it never really became the real genre with strict rules. The most important musicians that are considered to be creators of free jazz are saxophonists Ornette Coleman, John Coltrane, Eric Dolphy, Albert Ayler, pianist Cecile Taylor and double bassist Charles Mingus.